Week 11Linux InternalsProcesses, schedulingLecture o.docx

Week 11
Linux Internals
Processes, scheduling
Lecture organization
Kernel Structure
Process structure
Process creation
Signals
Process management, scheduling
Linux for embedded systems
References
Linux Internals (to the power of -1)
Simone Demblon, Sebastian Spitzner
http://www.tutorialized.com/view/tutorial/Linux-kernel-
internals-from-Process-Birth-to-Death/40955
Linux Knowledge Base and Tutorial
http://linux-
tutorial.info/modules.php?name=MContent&pageid=224
Inside the Linux scheduler IBM Developer Works
http://www.ibm.com/developerworks/linux/library/l-scheduler/
The Linux Kernel
A Unix kernel fulfills 4 main management tasks:

• Memory management
• Process management
• File system management
• IO management
For our study of real time implications, we will examine
Process management
A Rather brief look at the kernel
Part 1
Process structure
Process data structure
A process is represented by a rather large structure called
task_struct.
It contains all of the necessary data to represent the process,
along with data for accounting and to maintain relationships
with other processes (parents and children).
The actual structure of the task_struct is many pages long
A short sample of task_struct is shown in the next slide

Task_struct
Pointers to open files
Memory map
Signals: received, masked
Register contents
Everything defining the state of the computation
Task_struct detail
The sample contains the state of execution, a stack, a set of
flags, the parent process, the thread of execution (of which
there can be many), and open files.
The state variable: the state of the task.
Typical states:
the process is running
In a run queue about to be running (TASK_RUNNING),
sleeping (TASK_INTERRUPTIBLE),
sleeping but unable to be woken up
(TASK_UNINTERRUPTIBLE),
stopped (TASK_STOPPED), or a few others.
Flags: the process is being created (PF_STARTING) or exiting
(PF_EXITING), or currently allocating memory
(PF_MEMALLOC).
The comm (command) field: the name of the executable
Priority: (called static_prio). The actual priority is determined

dynamically based on loading and other factors.
More Task_struct
The tasks field is a linked-list
mm and active_mm fields The process's address space. mm
represents the process's memory descriptors, while the
active_mm is the previous process's memory descriptors
The thread_struct identifies the stored state of the process: The
CPU state (hardware registers, program counter, etc.).
Part 2
Process creation
System call functions
user-space tasks and kernel tasks, rely on a function called
do_fork to create the new process.
In the case of creating a kernel thread, the kernel calls a
function called kernel_thread
In user-space, a program calls fork, which results in a system
call to the kernel function called sys_fork
The function relationships are shown graphically in the next
slide.
Function hierarchy for process creation

do_fork
The do_fork function begins with a call to alloc_pidmap, which
allocates a new PID.
The do_fork function then calls copy_process, passing the flags,
stack, registers, parent process, and newly allocated PID.
The copy_process function is where the new process is created
as a copy of the parent. This function performs all actions
except for starting the process
Copying the process
Next, dup_task_struct allocates a new task_struct and copies the
current process's descriptors into it.
After a new thread stack is set up, control returns to
copy_process. A sequence of copy functions is then invoked to
copy, open file descriptors, signal information, process memory
and finally the thread.
The new task is then assigned to a processor. The priority of the
new process inherits the priority of the parent, and control
returns to do_fork. At this point, your new process exists but is
not yet running.
The do_fork function fixes this with a call to
wake_up_new_task. This function places the new process in a
run queue, then wakes it up for execution.

Finally, upon returning to do_fork, the PID value is returned to
the caller and the process is complete.
Part 3
Process management
Process Management
A program is an executable file, whereas a process is a running
program.
A process is an instance of a program in memory, executing
instructions on the processor.
The only way to run a program on a Unix/Linux system is to
request the kernel to execute it via an exec() system call.
Remember that the only things that can make system calls are
processes (binary programs that are executing.)
Scheduler
The scheduler manages processes and the fair distribution of
CPU time between them.
The kernel classifies processes as being in one of two possible
queues at any given time: the sleep queue and the run queue.
Run Queue
Processes in the run queue compete for access to the CPU.
The processes in the run queue compete for the processor(s). It
is the schedulers' job to allocate a time slice to each process,
and to let each process run on the processor for a certain
amount of time in turn.

Each time slice is so short (fractions of a second) that each
process in the run queue gets to run often every second. It
appears as though all of these processes are "running at the
same time". This is called round robin scheduling.
On a uniprocessor system, only one process can execute
instructions at any one time.
Only on a multiprocessor system can true multiprocessing
occur, with more than one process (as many as there are CPUs)
executing instructions simultaneously.
Classes of scheduling
There are different classes of scheduling besides round-robin.
An example would be real-time scheduling.
Different Unix systems have different scheduling classes and
features, and Linux is no exception.
Sleep queue
Processes waiting for a resource wait on the sleep queue. These
processes do not take up a slot on the run queue
Once the resource becomes available, it is reserved by the
process, which is then moved back onto the run queue to wait
for a turn on the processor.
Every process waiting for a resource is placed on the sleep
queue, including new processes, whose resources still have to
be allocated, even if they are readily available.

Queue management
Time-slicing
Each 10 milli-seconds (This may change with the HZ value) an
Interrupt comes on IRQ0, which helps us in a multitasking
environment.
The interrupt signal to the CPU comes from PIC (Programmable
Interrupt Controller).
With each Time-slice the current process execution is
interrupted (without task switching), and the processor does
housekeeping then the current process is re-activated unless a
higher priority process is waiting.
When does switching occur?
Some examples are:
• When a Time Slice ends the scheduler gives access to another
process
• If needing a resource, the process will have to go back into the
sleep queue to wait for or to be given access to that resource,
and only then would it be ready to be scheduled access to the
processor again.
• If we have a process waiting for information from another
process in the form of piped information. That process would
have to run before this process can continue, so the other

process would be given priority for the processor.
Process destruction
Process destruction can be driven by several events:
normal process termination,
through a signal
through a call to the exit function.
However process exit is driven, the process ends through a call
to the kernel function do_exit
This process is shown graphically in the next slide
Function hierarchy for process destruction
do_exit
do_exit removes all references to the current process from the
operating system
The cycle of detaching the process from the various resources
that it attained during its life is performed through a series of
calls, including exit_mm (to remove memory pages) to
exit_keys (which disposes of per-thread session and process
security keys).
The do_exit function performs various accountings for the
disposal of the process, then a series of notifications
Finally, the process state is changed to PF_DEAD, and the
schedule function is called to select a new process to execute.
Time slice

When a process's time slice has run out or for some other reason
another process gets to run, it is suspended (placed on the sleep
or run queue).
It’s state is stored in task_struct, so that when it gets a turn
again. This enables the process to return to the exact place
where it left off.
System Processes
In addition to user processes, there are system processes
running.
Some deal with managing memory and scheduling turns on the
CPU. Others deal with delivering mail, printing.
In principle, both of these kinds of processes are identical.
However, system processes can run at much higher priorities
and therefore run more often than user processes.
Typically a system process of this kind is referred to as a
daemon process because they run without user intervention.
Filesystems
Under Linux, files and directories are grouped into units called
filesystems. Filesystems exist within a section of the hard disk
called a partition.
Each hard disk can be broken down into multiple partitions and
a filesystem is created within the partition. (Some dialects of
UNIX allow multiple filesystems within a partition.)
The Life Cycle of Processes

http://linux-
tutorial.info/modules.php?name=MContent&pageid=84
From the time a process is created with a fork() until it has
completed its job and disappears from the process table, it goes
through many different states. The state a process is in changes
many times during its "life." These changes can occur, for
example, when the process makes a system call, it is someone
else's turn to run, an interrupt occurs, or the process asks for a
resource that is currently not available.
A commonly used model shows processes operating in one of
six separate states:
executing in user mode
executing in kernel mode
ready to run
sleeping
newly created, not ready to run, and not sleeping
issued exit system call (zombie)
Device Files
In Linux, almost everything is either treated as a file or is only
accessed through files.
For example, to read the contents of a data file, the operating
system must access the hard disk. Linux treats the hard disk as
if it were a file. It opens it like a file, reads it like a file, and
closes it like a file. The same applies to other hardware such as
tape drives and printers. Even memory is treated as a file. The
files used to access the physical hardware are device files.
When the Linux wants to access any hardware device, it first
opens a file that "points" toward that device (the device node).
Based on information it finds in the inode, the operating system
determines what kind of device it is and can therefore access it

in the proper manner. This includes opening, reading, and
closing, just like any other file.
Process states
State Diagram
State transitions
A newly created process enters the system in state 5.
If an exec() is made, then this process will end up in kernel
mode (2).
When a process is running, an interrupt may be generated (more
often than not, this is the system clock) and the currently
running process is pre-empted (3).
This is the same state as state 3 because it is still ready to run
and in main memory.
More Transitions
When the process makes a system call while in user mode (1), it
moves into state 2 where it begins to run in kernel mode.
Assume at this point that the system call made was to read a file
on the hard disk. Because the read is not carried out
immediately, the process goes to sleep, waiting on the event
that the system has read the disk and the data is ready. It is now
in state 4.

When the data is ready, the process is awakened. This does not
mean it runs immediately, but rather it is once again ready to
run in main memory (3).
If a process that was asleep is awakened (perhaps when the data
is ready), it moves from state 4 (sleeping) to state 3 (ready to
run). This can be in either user mode (1) or kernel mode (2).
End
A process can end its life by either explicitly calling the exit()
system call or having it called for them.
The exit() system call releases all the data structures that the
process was using.
One exception is the slot in the process table, which is the
responsibility of the init process. The slot in the process table is
used for the exit code of the exiting process. This can be used
by the parent process to determine whether the process did what
it was supposed to do or whether it ran into problems.
The process shows that it has terminated by putting itself into
state 8, and it becomes a "zombie." Once here, it can never run
again because nothing exists other than the entry in the process
table.
This is why you cannot "kill" a zombie process. There is
nothing there to kill. To kill a process, you need to send it a
signal (more on signals later). Because there is nothing there to
receive or process that signal, trying to kill it makes little sense.
The only thing to do is to let the system clean it up.

Access to a resource
Processes waiting for a common resource all wait on the same
wait channel. All are woken up when the resource is ready.
Only one process gets the resource. The others put themselves
back to sleep on the same wait channel.
When a process puts itself to sleep, it voluntarily gives up the
CPU. It may be that this process had just started its turn when it
couldn’t access the resource it needed.
Signals
Signals are a way of sending simple messages to processes.
Most of these messages are already defined and can be found in
<linux/signal.h>. Most are generated in the kernel
Signals can only be processed when the process is in user
mode. If a signal has been sent to a process that is in kernel
mode, it is dealt with immediately on returning to user mode.
Signals -2
Signals are used to signal asynchronous events to one or more
processes. A signal could be generated by a keyboard interrupt
or an error condition such as the process attempting to access a
non-existent location in its virtual memory.
Signals are also used by the shells to signal job control
commands to their child processes.
There are a set of defined signals that the kernel can generate or
that can be generated by other processes in the system, provided
that they have the correct privileges.

Signals - 3
Linux implements signals using information stored in the
task_struct for the process. The number of supported signals is
limited to the word size of the processor.
The currently pending signals are kept in the signal field with a
mask of blocked signals held in blocked. With the exception of
SIGSTOP and SIGKILL, all signals can be blocked.
If a blocked signal is generated, it remains pending until it is
unblocked.
Linux also holds information about how each process handles
every possible signal and this is held in an array of sigaction
data structures pointed at by the task_struct for each process.
Amongst other things it contains either the address of a routine
that will handle the signal or a flag which tells Linux that the
process either wishes to ignore this signal or let the kernel
handle the signal for it.
The process modifies the default signal handling by making
system calls and these calls alter the sigaction for the
appropriate signal as well as the blocked mask.
Problems with earlier Linux schedulers
Before the 2.6 kernel, the scheduler had a significant limitation
when many tasks were active. This was due to the scheduler
being implemented using an algorithm with O(n) complexity. In
other words, the more tasks (n) are active, the longer it takes to
schedule a task. At very high loads, the processor can be
consumed with scheduling and devote little time to the tasks
themselves.

The pre-2.6 scheduler also used a single runqueue for all
processors in a symmetric multiprocessing system (SMP). This
meant a task could be scheduled on any processor -- which can
be good for load balancing but bad for memory caches
Finally, preemption wasn't possible in the earlier scheduler; this
meant that a lower priority task could execute while a higher
priority task waited for it to complete.
Introducing the Linux 2.6 scheduler
The 2.6 scheduler resolves the primary three issues found in the
earlier scheduler (O(n) and SMP scalability issues), as well as
other problems. Now we'll explore the basic design of the 2.6
scheduler.
Major scheduling structures
Each CPU has a runqueue made up of 140 priority lists that are
serviced in FIFO order.
Tasks that are scheduled to execute are added to the end of their
respective runqueue's priority list.
Each task has a time slice that determines how much time it's
permitted to execute.
The first 100 priority lists of the runqueue are reserved for real-
time tasks, and the last 40 are used for user tasks (see next
slide).
Priority lists
expired active
Runqueues
In addition to the CPU's runqueue, which is called the active

runqueue, there's also an expired runqueue.
When a task on the active runqueue uses all of its time slice, it's
moved to the expired runqueue. During the move, its time slice
is recalculated.
If no tasks exist on the active runqueue for a given priority, the
pointers for the active and expired runqueues are swapped, thus
making the expired priority list the active one.
The job of the scheduler is simple: choose the task on the
highest priority list to execute.
http://www.linuxjournal.com/article/7477
By Aseem R. Deshpande
With the release of kernel 2.6, Linux now poses serious
competition to major RTOS vendors in the embedded market
space.
Linux 2.6 introduces many new features that make it an
excellent operating system for embedded computing.
Among these new features are enhanced real-time performance,
easier porting to new computers, support for large memory
models, support for microcontrollers and an improved I/O
system.
Characteristics of Embedded Systems
Embedded systems often need to meet timing constraints
reliably.
,embedded systems have access to far fewer resources than does
a normal PC. They have to squeeze maximum value out of
whatever is available.

. The OS should perform reliably and efficiently, if possible,
under the cases of extreme load.
If a crash occurs in one part of the module, it should not effect
other parts of the system. Furthermore, recovery from crashes
should be graceful.
How Linux 2.6 Satisfies the Requirements
Although Linux 2.6 is not yet a true real-time operating system,
it does contain improvements that make it a worthier platform
than previous kernels when responsiveness is desirable.
Three significant improvements are preemption points in the
kernel, an efficient scheduler and improved synchronization.
Kernel Preemption
As with most general-purpose operating systems, Linux always
has forbidden the process scheduler from running when a
process is executing in a system call. Therefore, once a task is
in a system call, that task controls the processor until the
system call returns, no matter how long that might take.
As of kernel 2.6, the kernel is preemptible. A kernel task now
can be preempted, so that some important user application can
continue to run.
In Linux 2.6, kernel code has been laced with preemption
points, instructions that allow the scheduler to run and possibly
block the current process so as to schedule a higher priority
process.
Kernel Preemption

Linux 2.6 avoids unreasonable delays in system calls by
periodically testing a preemption point. During these tests, the
calling process may block and let another process run.
Thus, under Linux 2.6, the kernel now can be interrupted mid-
task, so other applications can continue to run even when
something low-level and complicated is going on in the
background.
Embedded software often has to meet deadlines that renders it
incompatible with virtual memory demand paging, in which
slow handling of page faults would ruin responsiveness.
To eliminate this problem, the 2.6 kernel can be built with no
virtual memory system. Of course, it then becomes the software
designer's responsibility to ensure enough real memory always
is available to get the job done.
An Efficient Scheduler
The process scheduler has been rewritten in the 2.6 kernel to
eliminate the slow algorithms of previous versions.
Formerly, in order to decide which task should run next, the
scheduler had to look at each ready task and make a
computation to determine that task's relative importance.
After all computations were made, the task with the highest
score would be chosen.
Because the time required for this algorithm varied with the
number of tasks, complex multitasking applications suffered
from slow scheduling.

2.6 improvements
The scheduler in Linux 2.6 no longer scans all tasks every time.
Instead, when a task becomes ready to run, it is sorted into
position on a queue, called the current queue.
Then, when the scheduler runs, it chooses the task at the most
favorable position in the queue. As a result, scheduling is done
in a constant amount of time.
When the task is running, it is given a time slice, or a period of
time in which it may use the processor, before it has to give
way to another thread.
When its time slice has expired, the task is moved to another
queue, called the expired queue.
The task is sorted into this expired queue according to its
priority. This new procedure is substantially faster than the old
one, and it works equally as well whether there are many tasks
or only a few in queue. This new scheduler is called the O(1)
scheduler.
New Synchronization Primitives
Applications involving the use of shared resources, such as
shared memory or shared devices, have to be developed
carefully to avoid race conditions.
The solution implemented in Linux, called Mutex, ensured that
only one task is using the resource at a time. Mutex involved a
system call to the kernel to decide whether to block the thread
or allow it to continue executing.
But when the decision is to continue, the time-consuming

system call was unnecessary. The new implementation in Linux
2.6 supports Fast User-Space Mutexes (Futex). These functions
can check from user space whether blocking is necessary and
perform the system call to block the thread only when it is
required.
When blocking is not required, avoiding the unneeded system
call saves time. It also supports setting priorities to allow
applications or threads of higher priority to have first access to
the contested resource.
Improved Threading Model and Support for NPTL
threading in 2.6 is based on a 1:1 threading model, one kernel
thread for one user thread. It also includes in-kernel support for
the new Native Posix Threading Library (NPTL).
NPTL brings an eight-fold improvement over its predecessor.
Tests conducted by its authors have shown that Linux, with this
new threading, can start and stop 100,000 threads
simultaneously in about two seconds. This task took 15 minutes
on the old threading model.
Along with POSIX threads, 2.6 provides POSIX signals and
POSIX high-resolution timers as part of the mainstream kernel.
POSIX signals are an improvement over UNIX-style signals,
which were the default in previous Linux releases. Unlike UNIX
signals, POSIX signals cannot be lost and can carry information
as an argument. Also, POSIX signals can be sent from one
POSIX thread to another, rather than only from process to
process, like UNIX signals.
Embedded systems often need to poll hardware or do other tasks
on a fixed schedule. POSIX timers make it easy to arrange any
task to be scheduled periodically. The clock that the timer uses
can be set to tick at a rate as fine as one kilohertz, so software

engineers can control the scheduling of tasks with precision.
Subarchitecture Support
Hardware designs in the embedded world often are customized
for special applications
For example, a purpose-built board may use different IRQ
management than what a similar reference design uses.
In order to run on the new board, Linux has to be ported or
altered to support the new hardware. This porting is easier if the
operating system is made of components that are well separated,
making it necessary to change only the code that has to change.
The components of Linux 2.6 that are likely to be altered for a
custom design have been refactored with a concept called
Subarchitecture. Components are separated clearly and can be
modified or replaced individually with minimal impact on other
components of the board support package.
By formalizing Linux's support for the slightly different
hardware types, the kernel can be ported more easily to other
systems, such as dedicated storage hardware and other
components that use industry-dominant processor types.
A small portion of task_struct
struct task_struct {
volatile long state;
void *stack;
unsigned int flags;
int prio, static_prio;
struct list_head tasks;

struct mm_struct *mm, *active_mm;
pid_t pid;
pid_t tgid;
struct task_struct *real_parent;
char comm[TASK_COMM_LEN];
struct thread_struct t hread;
struct files_struct *files;
...
};
The states listed here describe what is happening conceptually
and do not indicate what "official"
state a process is in. The official states are listed below:
TASK_RUNNING task (process) currently running
TASK_INTERRUPTABLE process is sleeping but can b e
woken up (interrupted)
TASK_UNINTERRUPTABLE process is sleeping but can not
be woken up (interrupted)
TASK_ZOMBIE process terminated but its status was not
collected (it was not waited for)
TASK_STOPPED process stopped by a debugger or job control
TASK_SWAPPING (removed in 2.3.x kernel)
Table - Process States in sched.h
week 12
Real time kernel

Ref: Extracts from:
http://www.swd.de/documents/manuals/sysarch/intro_en.html#id
1
The QNX Operating System
QNX provides multitasking, priority-driven preemptive
scheduling, and fast context switching
QNX is flexible. It can be customized from a ``bare-bones''
kernel with a few small modules to a network-wide system
equipped to serve hundreds of users.
QNX achieves this through two fundamental principles:
microkernel architecture
message-based interprocess communication
QNX's microkernel architecture
Architecture
QNX Microkernel is dedicated to only two essential functions:
message passing - the Microkernel handles the routing of all
messages among all processes throughout the entire system
scheduling - the scheduler is a part of the Microkernel and is
invoked whenever a process changes state as the result of a

message or interrupt
Unlike processes, the Microkernel itself is never scheduled for
execution. It is entered only as the direct result of kernel calls,
either from a process or from a hardware interrupt
QNX configuration
A typical QNX configuration has the following system
processes:
Process Manager (Proc)
Filesystem Manager (Fsys)
Device Manager (Dev)
Network Manager (Net)
System processes vs. user-written processes
System processes are like user processes: they have no private
or hidden interfaces that are unavailable to user processes.
This gives QNX unparalleled extensibility: the OS itself can be
augmented by user written programs.
The only real difference between system services and
applications is that OS services manage resources for clients.
Drivers
Since drivers start up as standard processes, adding a new driver
to QNX doesn't affect any other part of the operating system.
The only change you need to make to your QNX environment is
to actually start the new driver.
Once they've completed their initialization, drivers can do
either of the following:
choose to disappear as standard processes, simply becoming

extensions to the system process they're associated with
retain their individual identity as standard processes
QNX as a message-passing operating system
In QNX, a message is a packet of bytes passed from one process
to another.
the data in a message has meaning for the sender of the message
and for its receiver, but for no one else.
Message passing not only allows processes to pass data to each
other, but also provides a means of synchronizing the execution
of several processes.
As they send, receive, and reply to messages, processes undergo
various ``changes of state'' that affect when, and for how long,
they may run.
Knowing their states and priorities, the Microkernel can
schedule all processes as efficiently as possible to make the
most of available CPU resources. This single, consistent method
- message-passing - is thus constantly operative throughout the
entire system.
The Microkernel
The QNX Microkernel is responsible for :
IPC
the Microkernel supervises the routing of messages; it also
manages two other forms of IPC: proxies and signals
low-level network communication
The Microkernel delivers all messages destined for processes on
other nodes
process scheduling
the Microkernel's scheduler decides which process will execute

next
first-level interrupt handling
all hardware interrupts and faults are first routed through the
Microkernel, then passed on to the appropriate driver or system
manager
Microkernel
Process synchronization
Message passing not only allows processes to pass data to each
other, but also provides a means of synchronizing the execution
of several cooperating processes.
Send() request, and the message it has sent hasn't yet been
received by the recipient processSEND-blockedSend() request,
and the message has been received by the recipient process, but
that process hasn't yet repliedREPLY-blockedReceive() request,
but hasn't yet received a messageRECEIVE-blocked
Send and Reply Blocking

IPC via proxies
A proxy is a form of non-blocking message especially suited for
event notification where the sending process doesn't need to
interact with the recipient
By using a proxy, a process or an interrupt handler can send a
message to another process without blocking or having to wait
for a reply.
Here are some examples of when proxies are used:
A process wants to notify another process that an event has
occurred,
A process wants to send data to another process, but needs
neither a reply nor any other acknowledgment that the recipient
has received the message.
An interrupt handler wants to tell a process that some data is
available for processing
IPC via signals
Signals are a traditional method of asynchronous
communication that have been available for many years in a
variety of operating systems.
QNX supports a rich set of POSIX-compliant signals, some
historical UNIX signals, as well as some QNX-specific signals.
Receiving signals
A process can receive a signal in one of three ways:
The default action for the signal is taken - usually, this default
action is to terminate the process.
The process can ignore the signal.

The process can provide a signal handler for the signal
Signals are delivered to a process when the process is made
ready to run by the Microkernel's scheduler.
Scheduling methods
QNX provides three scheduling methods:
FIFO scheduling
round-robin scheduling
adaptive scheduling
They are effective on a per-process basis, not on a global basis
for all processes on a node.
These methods apply only when two or more processes that
share the same priority are READY. If a higher-priority process
becomes READY, it immediately preempts all lower-priority
processes.
Realtime performance
Interrupt latency is the time from the reception of a hardware
interrupt until the first instruction of a software interrupt
handler is executed. QNX leaves interrupts fully enabled almost
all the time, so that interrupt latency is typically insignificant.
But certain critical sections of code do require that interrupts be
temporarily disabled. The maximum such disable time usually
defines the worst-case interrupt latency - in QNX this is very
small.
hardware interrupt is processed by an established interrupt
handler.
The interrupt handler either will simply return, or it will return

and cause a proxy to be triggered.
Scheduling latency
In some cases, the low-level hardware interrupt handler must
schedule a higher-level process to run. In this scenario, the
interrupt handler will return and indicate that a proxy is to be
triggered. This introduces a second form of latency - scheduling
latency - which must be accounted for.
Scheduling latency is the time between the termination of an
interrupt handler and the execution of the first instruction of a
driver process.
This usually means the time it takes to save the context of the
currently executing process and restore the context of the
required driver process. Although larger than interrupt latency,
this time is also kept small in a QNX system.
Process Manager responsibilities
The Process Manager works with the Microkernel to provide
essential operating system services. Although it shares the same
address space as the Microkernel, it runs as a true process.
It is scheduled to run by the Microkernel like other processes

and it uses the Microkernel's message-passing primitives to
communicate with other processes in the system.
It is responsible for creating new processes in the system and
managing the resources associated with a process. These
services are all provided via messages
Process creation primitives
QNX supports three process-creation primitives:
fork()
exec()
spawn()
Both fork() and exec() are defined by POSIX, while the
implementation of spawn() is unique to QNX.
The spawn() primitive creates a new process as a child of the
calling process.
Process states
Interrupt handlers
Interrupt handlers react to hardware interrupts and manage the
low-level transfer of data between the computer and external
devices.
Interrupt handlers are physically packaged as part of a standard
QNX process (e.g. a driver), but they always run
asynchronously to the process they're associated with.

An interrupt handler:
is entered with a far call, not directly from the interrupt itself
(this can be written in C, rather than in assembler)
runs in the context of the process it is embedded in, so it has
access to all the global variables of the process
runs with interrupts enabled, but is preempted only if a higher-
priority interrupt occurs
shouldn't talk directly to the 8259 interrupt hardware (the
operating system takes care of this)
should be as short as possible.
1
Free Electrons. Kernel, drivers and embedded Linux developme
nt, consulting, training and support. http://free-electrons.com
Embedded Linux system development
Real-time in
embedded Linux
systems
Michael Opdenacker
Thomas Petazzoni
Gilles Chanteperdrix
Free Electrons
© Copyright 2004-2011, Free Electrons.
Creative Commons BY-SA 3.0 license
Latest update: Feb 21, 2011,
Document sources, updates and translations:

http://free-electrons.com/docs/realtime
Corrections, suggestions, contributions and translations are wel
come!
http://free-electrons.com/docs/realtime
2
Real Time in Embedded Linux Systems
Introduction
3
Embedded Linux and real time
Due to its advantages, Linux and the open-source softwares are
more and more commonly used in embedded applications
However, some applications also have real-time constraints
They, at the same time, want to
Get all the nice advantages of Linux: hardware support,
components re-use, low cost, etc.
Get their real-time constraints met
?

4
Embedded Linux and real time
Linux is an operating system part of the large Unix family
It was originally designed as a time-sharing system
The main goal is to get the best throughput from the available
hardware, by making the best possible usage of resources (CPU,
memory, I/O)
Time determinism is not taken into account
On the opposite, real-time constraints imply time determinism,
even at the expense of lower global throughput
Best throughput and time determinism are contradictory
requirements
5
Linux and real-time approaches
Over time, two major approaches have been taken to bring real-
time requirements into Linux

Approach 1
Improve the Linux kernel itself so that it matches real-time
requirements, by providing bounded latencies, real-time APIs, e
tc.
Approach taken by the mainline Linux kernel and the
PREEMPT_RT project.
Approach 2
Add a layer below the Linux kernel that will handle all the real-
time
requirements, so that the behaviour of Linux doesn't affect real-
time
tasks.
Approach taken by RTLinux, RTAI and Xenomai
6
Approach 1
Improving the main Linux kernel with
PREEMPT_RT
7

Understanding latency
When developing real-time applications with a system such as
Linux, the typical scenario is the following
An event from the physical world happens and gets notified to t
he
CPU by means of an interrupt
The interrupt handler recognizes and handles the event, and the
n
wake-up the user-space task that will react to this event
Some time later, the user-space task will run and be able to reac
t to
the physical world event
Real-time is about providing guaranteed worst case latencies for
this reaction time, called latency
Something not very important...
Your important
real-time task !
Interrupt ! ?
8
Linux kernel latency components

Waiting
task
interrupt
latency
Interrupt
handler Scheduler
Running task
Interrupt
Scheduling latency
scheduler
latency
scheduler
duration
Process
context
Interrupt
context
Makes the
task runnable
kernel latency = interrupt latency + handler duration
+ scheduler latency + scheduler duration
handler
duration

9
Interrupt latency
Waiting
task
interrupt
latency
Interrupt
handler Scheduler
Running task
Interrupt
handler
duration
scheduler
latency
scheduler
duration
Makes the
task runnable
Time elapsed before executing the interrupt handler
Scheduling latency

10
Source of interrupt latency
One of the concurrency prevention mechanism used in the kerne
l
is the spinlock
It has several variants, but one of the variant commonly used to
prevent concurrent accesses between a process context and an
interrupt context works by disabling interrupts
Critical sections protected by spinlocks, or other section in whic
h
interrupts are explictly disabled will delay the beginning of the
execution of the interrupt handler
The duration of these critical sections is unbounded
Other possible source: shared interrupts
Kernel
code
Critical section
protected by spinlock
Interrupt
handler
Interrupt ?

11
Interrupt handler duration
Waiting
task
interrupt
latency
Interrupt
handler Scheduler
Running task
Interrupt
handler
duration
scheduler
latency
scheduler
duration
Makes the
task runnable
Time taken to execute the interrupt handler

Scheduling latency
12
Interrupt handler implementation
In Linux, many interrupt handlers are split in two parts
A top-half, started by the CPU as soon as interrupt are
enabled. It runs with the interrupt line disabled and is
supposed to complete as quickly as possible.
A bottom-half, scheduled by the top-half, which starts after all
pending top-half have completed their execution.
Therefore, for real-time critical interrupts, bottom-half
shouldn't be used: their execution is delayed by all other
interrupts in the system.
Top half
Interrupt ACK Exit
Bottom half
Schedule
bottom
half
Other interrupt
handlers...

Handle
device
data...
Wake up
waiting
tasks
User space...
13
Scheduler latency
Waiting
task
interrupt
latency
Interrupt
handler Scheduler
Running task
Interrupt
handler
duration
scheduler
latency

scheduler
duration
Makes the
task runnable
Time elapsed before executing the scheduler
Scheduling latency
14
Understanding preemption (1)
The Linux kernel is a preemptive operating system
When a task runs in user-space mode and gets interrupted by an
interruption, if the interrupt handler wakes up another task, this
task can be scheduled as soon as we return from the interrupt
handler.
Task A
(running in user mode)
Interrupt handler
Wakes up Task B
Task B
(running in user mode)
Interrupt

15
Understanding preemption (2)
However, when the interrupt comes while the task is executing a
system call, this system call has to finish before another task ca
n
be scheduled.
By default, the Linux kernel does not do kernel preemption.
This means that the time before which the scheduler will be
called to schedule another task is unbounded.
Task A
(user mode)
Interrupt handler
Wakes up Task B
Task B
(user mode)
System call
Task A
(kernel mode)
Task A
(kernel mode)

Interrupt
?
Return from syscall
16
Scheduler duration
Waiting
task
interrupt
latency
Interrupt
handler Scheduler
Running task
Interrupt
handler
duration
scheduler
latency
scheduler
duration

Makes the
task runnable
Time taken to execute the scheduler
and switch to the new task.
Scheduling latency
17
Other non-deterministic mechanisms
Outside of the critical path detailed previously, other non-
deterministic mechanisms of Linux can affect the execution tim
e
of real-time tasks
Linux is highly based on virtual memory, as provided by an MM
U,
so that memory is allocated on demand. Whenever an applicatio
n
accesses code or data for the first time, it is loaded on demand,
which can creates huge delays.
Many C library services or kernel services are not designed with
real-time constraints in mind.
18

Priority inversion
Acquires
a lock
Priority
Time
preempted
Tries to get
the same
lock
waits
A process with a low priority might hold a lock needed by a hig
her
priority process, effectively reducing the priority of this process
.
Things can be even worse if a middle priority process uses the
CPU.
19
Interrupt handler priority
top priority task

Any interrupt
top priority task
Any interrupt...
In Linux, interrupt handlers are executed directly by the CPU
interrupt mechanisms, and not under control of the Linux
scheduler. Therefore, all interrupt handlers have an higher
priority than all tasks running on the system.
20
The PREEMPT_RT project
Long-term project lead by Linux kernel developers Ingo Molnar
,
Thomas Gleixner and Steven Rostedt
https://rt.wiki.kernel.org
The goal is to gradually improve the Linux kernel regarding real
-
time requirements and to get these improvements merged into
the mainline kernel
PREEMPT_RT development works very closely with the mainli
ne
development
Many of the improvements designed, developed and debugged
inside PREEMPT_RT over the years are now part of the mainlin

e
Linux kernel
The project is a long-term branch of the Linux kernel that ultim
ately
should disappear as everything will have been merged
https://rt.wiki.kernel.org/
21
Improvements in the mainline kernel
Coming from the
PREEMPT_RT project
Since the beginning of 2.6
O(1) scheduler
Kernel preemption
Better POSIX real-time API
support
Since 2.6.18
Priority inheritance support
for mutexes
Since 2.6.21
High-resolution timers

Since 2.6.30
Threaded interrupts
Since 2.6.33
Spinlock annotations
22
New preemption options in Linux 2.6
2 new preemption models offered by standard Linux 2.6:
23
1st option: no forced preemption
CONFIG_PREEMPT_NONE
Kernel code (interrupts, exceptions, system calls) never preempt
ed.
Default behavior in standard kernels.
Best for systems making intense computations,
on which overall throughput is key.
Best to reduce task switching to maximize CPU and cache usage

(by reducing context switching).
Still benefits from some Linux 2.6 improvements:
O(1) scheduler, increased multiprocessor safety (work on RT
preemption was useful to identify hard to find SMP bugs).
Can also benefit from a lower timer frequency
(100 Hz instead of 250 or 1000).
24
2nd option: voluntary kernel preemption
CONFIG_PREEMPT_VOLUNTARY
Kernel code can preempt itself
Typically for desktop systems, for quicker application reaction t
o
user input.
Adds explicit rescheduling points throughout kernel code.
Minor impact on throughput.
25
3rd option: preemptible kernel

CONFIG_PREEMPT
Most kernel code can be involuntarily preempted at any time.
When a process becomes runnable, no more need to wait for
kernel code (typically a system call) to return before running th
e
scheduler.
Exception: kernel critical sections (holding spinlocks), but a
rescheduling point occurs when exiting the outer critical section
,
in case a preemption opportunity would have been signaled whil
e
in the critical section.
Typically for desktop or embedded systems with latency
requirements in the milliseconds range.
Still a relatively minor impact on throughput.
26
Priority inheritance
One classical solution to the priority inversion problem is called
priority inheritance
The idea is that when a task of a low priority holds a lock reque
sted
by an higher priority task, the priority of the first task gets temp
orarly
raised to the priority of the second task : it has inherited its prio

rity.
In Linux, since 2.6.18, mutexes support priority inheritance
In userspace, priority inheritance must be explictly enabled on a
per-mutex basis.
27
High resolution timers
The resolution of the timers used to be bound to the resolution o
f
the regular system tick
Usually 100 Hz or 250 Hz, depending on the architecture and th
e
configuration
A resolution of only 10 ms or 4 ms.
Increasing the regular system tick frequency is not an option as
it
would consume too much resources
The high-resolution timers infrastructure, merged in 2.6.21,
allows to use the available hardware timers to program interrupt
s
at the right moment.
Hardware timers are multiplexed, so that a single hardware time

r is
sufficient to handle a large number of software-programmed tim
ers.
Usable directly from user-space using the usual timer APIs
28
Threaded interrupts
To solve the interrupt inversion problem, PREEMPT_RT has
introduced the concept of threaded interrupts
The interrupt handlers run in normal kernel threads, so that the
priorities of the different interrupt handlers can be configured
The real interrupt handler, as executed by the CPU, is only in
charge of masking the interrupt and waking-up the correspondin
g
thread
The idea of threaded interrupts also allows to use sleeping
spinlocks (see later)
Merged since 2.6.30, the conversion of interrupt handlers to
threaded interrupts is not automatic : drivers must be modified
In PREEMPT_RT, all interrupt handlers are switched to threade
d
interrupts

29
PREEMPT_RT specifics
30
CONFIG_PREEMPT_RT (1)
The PREEMPT_RT patch adds a new « level » of preemption,
called CONFIG_PREEMPT_RT
This level of preemption replaces all kernel spinlocks by mutex
es
(or so-called sleeping spinlocks)
Instead of providing mutual exclusion by disabling interrupts an
d
preemption, they are just normal locks : when contention happe
ns,
the process is blocked and another one is selected by the schedu
ler
Works well with threaded interrupts, since threads can block, w
hile
usual interrupt handlers could not
Some core, carefully controlled, kernel spinlocks remain as nor
mal
spinlocks

31
CONFIG_PREEMPT_RT (2)
With CONFIG_PREEMPT_RT, virtually all kernel code become
s
preemptible
An interrupt can occur at any time, when returning from the inte
rrupt
handler, the woken up process can start immediately
This is the last big part of PREEMPT_RT that isn't fully in the
mainline kernel yet
Part of it has been merged in 2.6.33 : the spinlock annotations.
The
spinlocks that must remain as spinning spinlocks are now
differentiated from spinlocks that can be converted to sleeping
spinlocks. This has reduced a lot the PREEMPT_RT patch size !
32
Threaded interrupts
The mechanism of threaded interrupts in PREEMPT_RT is still
different from the one merged in mainline

In PREEMPT_RT, all interrupt handlers are unconditionally
converted to threaded interrupts.
This is a temporary solution, until interesting drivers in mainlin
e
get gradually converted to the new threaded interrupt API that
has been merged in 2.6.30.
33
Setting up PREEMPT_RT
34
PREEMPT_RT setup (1)
PREEMPT_RT is delivered as a patch against the mainline
kernel
Best to have a board supported by the mainline kernel, otherwis
e
the PREEMPT_RT patch may not apply and may require some
adaptations
Many official kernel releases are supported, but not all. For
example, 2.6.31 and 2.6.33 are supported, but not 2.6.32.

Quick set up
Download and extract mainline kernel
Download the corresponding PREEMPT_RT patch
Apply it to the mainline kernel tree
35
PREEMPT_RT setup (2)
In the kernel configuration, be sure to enable
CONFIG_PREEMPT_RT
High-resolution timers
Compile your kernel, and boot
You are now running the real-time Linux kernel
Of course, some system configuration remains to be done, in
particular setting appropriate priorities to the interrupt threads,
which depend on your application.
36

Real-time application development
37
Development and compilation
No special library is needed, the POSIX realtime API is part of
the standard C library
The glibc or eglibc C libraries are recommended, as the support
of some real-time features is not available yet in uClibc
Priority inheritance mutexes or NPTL on some architectures, for
example
Compile a program
ARCH-linux-gcc -o myprog myprog.c -lrt
To get the documentation of the POSIX API
Install the manpages-posix-dev package
Run man functioname
38

Process, thread ?
Confusion about the terms «process», «thread» and «task»
In Unix, a process is created using fork() and is composed of
An address space, which contains the program code, data, stack,
shared libraries, etc.
One thread, that starts executing the main() function.
Upon creation, a process contains one thread
Additional threads can be created inside an existing process,
using pthread_create()
They run in the same address space as the initial thread of the
process
They start executing a function passed as argument to
pthread_create()
39
Process, thread: kernel point of view
The kernel represents each thread running in the system by a
structure of type task_struct
From a scheduling point of view, it makes no difference betwee
n

the initial thread of a process and all additional threads created
dynamically using pthread_create()
Address space
Thread
A
Process after fork()
Address space
Thread
A
Thread
B
Same process after pthread_create()
40
Creating threads
Linux support the POSIX thread API
To create a new thread
pthread_create(pthread_t *thread,
pthread_attr_t *attr,
void *(*routine)(*void*),
void *arg);

The new thread will run in the same address space, but will be
scheduled independently
Exiting from a thread
pthread_exit(void *value_ptr);
Waiting for a thread termination
pthread_join(pthread_t *thread, void **value_ptr);
41
Scheduling classes (1)
The Linux kernel scheduler support different scheduling classes
The default class, in which processes are started by default is a
time-sharing class
All processes, regardless of their priority, get some CPU time
The proportion of CPU time they get is dynamic and affected by
the
nice value, which ranges from -20 (highest) to 19 (lowest). Can
be
set using the nice or renice commands
The real-time classes SCHED_FIFO and SCHED_RR
The highest priority process gets all the CPU time, until it block

s.
In SCHED_RR, round-robin scheduling between the processes o
f
the same priority. All must block before lower priority processe
s get
CPU time.
Priorities ranging from 0 (lowest) to 99 (highest)
42
An existing program can be started in a specific scheduling clas
s
with a specific priority using the chrt command line tool
Example: chrt -f 99 ./myprog
The sched_setscheduler() API can be used to change the
scheduling class and priority of a process
int sched_setscheduler(pid_t pid, int policy,
const struct sched_param *param);
policy can be SCHED_OTHER, SCHED_FIFO, SCHED_RR, etc
.
param is a structure containing the priority

43
The priority can be set on a per-thread basis when a thread is
created :
Then the thread can be created using pthread_create(),
passing the attr structure.
Several other attributes can be defined this way: stack size, etc.
struct sched_param parm;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setinheritsched(&attr,
PTHREAD_EXPLICIT_SCHED);
pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
parm.sched_priority = 42;
pthread_attr_setschedparam(&attr, &parm);
44
Memory locking
In order to solve the non-determinism introduced by virtual
memory, memory can be locked

Guarantee that the system will keep it allocated
Guarantee that the system has pre-loaded everything into memor
y
mlockall(MCL_CURRENT | MCL_FUTURE);
Locks all the memory of the current address space, for currently
mapped pages and pages mapped in the future
Other, less useful parts of the API: munlockall, mock,
munlock.
Watch out for non-currently mapped pages
Stack pages
Dynamically-allocated memory
45
Mutexes
Allows mutual exclusion between two threads in the same
address space
Initialization/destruction
pthread_mutex_init(pthread_mutex_t *mutex, const
pthread_mutexattr_t *mutexattr);
pthread_mutex_destroy(pthread_mutex_t *mutex);

Lock/unlock
pthread_mutex_lock(pthread_mutex_t *mutex);
pthread_mutex_unlock(pthread_mutex_t *mutex);
Priority inheritance must explictly be activated
pthread_mutexattr_t attr;
pthread_mutexattr_init (&attr);
pthread_mutexattr_getprotocol
(&attr, PTHREAD_PRIO_INHERIT);
46
Timers
timer_create(clockid_t clockid,
struct sigevent *evp,
timer_t *timerid)
Create a timer. clockid is usually CLOCK_MONOTONIC.
sigevent defines what happens upon timer expiration : send a
signal or start a function in a new thread. timerid is the returned
timer identifier.
timer_settime(timer_t timerid, int flags,
struct itimerspec *newvalue,
struct itimerspec *oldvalue)
Configures the timer for expiration at a given time.

timer_delete(timer_t timerid), delete a timer
clock_getres(), get the resolution of a clock
Other functions: timer_getoverrun(), timer_gettime()
47
Signals
Signals are an asynchronous notification mechanism
Notification occurs either
By the call of a signal handler. Be careful with the limitations o
f
signal handlers!
By being unblocked from the sigwait(), sigtimedwait() or
sigwaitinfo() functions. Usually better.
Signal behaviour can be configured using sigaction()
Mask of blocked signals can be changed with
pthread_sigmask()
Delivery of a signal using pthread_kill() or tgkill()
All signals between SIGRTMIN and SIGRTMAX, 32 signals un
der
Linux.

48
Inter-process communication
Semaphores
Usable between different processes using named semaphores
sem_open(), sem_close(), sem_unlink(), sem_init(),
sem_destroy(), sem_wait(), sem_post(), etc.
Message queues
Allows processes to exchange data in the form of messages.
mq_open(), mq_close(), mq_unlink(), mq_send(),
mq_receive(), etc.
Shared memory
Allows processes to communicate by sharing a segment of mem
ory
shm_open(), ftruncate(), mmap(), munmap(),
close(), shm_unlink()
49

Debugging real-time latencies
50
ftrace - Kernel function tracer
New infrastructure that can be used for debugging or analyzing
latencies and performance issues in the kernel.
Developed by Steven Rostedt. Merged in 2.6.27.
For earlier kernels, can be found from the rt-preempt patches.
Very well documented in Documentation/ftrace.txt
Negligible overhead when tracing is not enabled at run-time.
Can be used to trace any kernel function!
See our video of Steven's tutorial at OLS 2008:
http://free-electrons.com/community/videos/conferences/
http://free-electrons.com/community/videos/conferences/
51
Using ftrace
Tracing information available through the debugfs virtual fs
(CONFIG_DEBUG_FS in the Kernel Hacking section)

Mount this filesystem as follows:
mount -t debugfs nodev /debug
When tracing is enabled (see the next slides),
tracing information is available in /debug/tracing.
Check available tracers
in /debug/tracing/available_tracers
52
Scheduling latency tracer
CONFIG_SCHED_TRACER (Kernel Hacking section)
Maximum recorded time between waking up a top priority task
and its scheduling on a CPU, expressed in µs.
Check that wakeup is listed in
/debug/tracing/available_tracers
To select, reset and enable this tracer:
echo wakeup > /debug/tracing/current_tracer
echo 0 > /debug/tracing/tracing_max_latency
echo 1 > /debug/tracing/tracing_enabled
Let your system run, in particular real-time tasks.
Example: chrt -f 5 sleep 1
Disable tracing:
echo 0 > /debug/tracing/tracing_enabled

Read the maximum recorded latency and the corresponding trac
e:
cat /debug/tracing/tracing_max_latency
53
Useful reading
About real-time support in the standard Linux kernel
Internals of the RT Patch, Steven Rostedt, Red Hat, June 2007
http://www.kernel.org/doc/ols/2007/ols2007v2-pages-161-172.p
df
Definitely worth reading.
The Real-Time Linux Wiki: http://rt.wiki.kernel.org
“The Wiki Web for the CONFIG_PREEMPT_RT community,
and real-time Linux in general.”
Contains nice and useful documents!
See also our books page.
http://www.kernel.org/doc/ols/2007/ols2007v2-pages-161-
172.pdf
http://rt.wiki.kernel.org/
54
Approach 2

Real-time extensions to the Linux kernel
55
Linux real-time extensions
Three generations
RTLinux
RTAI
Xenomai
A common principle
Add a extra layer between the
hardware and the Linux kernel,
to manage real-time tasks
separately.
Hardware
Micro-kernel
Linux
kernel
real-time
tasks
real-time

tasks
56
RTLinux
First real-time extension for Linux, created by Victor Yodaiken.
Nice, but the author filed a software patent covering the additio
n of real-
time support to general operating systems as implemented in RT
Linux!
Its Open Patent License drew many developers away and frighte
ned
users. Community projects like RTAI and Xenomai now attract
most
developers and users.
February, 2007: RTLinux rights sold to Wind River.
Now supported by Wind River as “RealTime Core for Wind Ri
ver Linux.”
Free version still advertised by Wind River on http://www.rtlinu
xfree.com,
but no longer a community project.
http://www.rtlinuxfree.com/
57

RTAI
http://www.rtai.org/ - Real-Time Application Interface for Linu
x
Created in 1999, by Prof. Paolo Montegazza (long time
contributor to RTLinux), Dipartimento di Ingegneria
Aerospaziale Politecnico di Milano (DIAPM).
Community project. Significant user base.
Attracted contributors frustrated by the RTLinux legal issues.
Only really actively maintained on x86
May offer slightly better latencies than Xenomai, at the
expense of a less maintainable and less portable code base
Since RTAI is not really maintained on ARM and other
embedded architectures, our presentation is focused on
Xenomai.
http://www.rtai.org/
58
Xenomai project
http://www.xenomai.org/
Started in 2001 as a project aiming at emulating
traditional RTOS.

Initial goals: facilitate the porting of programs to GNU / Linux.
Initially related to the RTAI project (as the RTAI / fusion
branch), now independent.
Skins mimicking the APIs of traditional
RTOS such as VxWorks, pSOS+, and VRTXsa as well as the
POSIX API, and a “native” API.
Aims at working both as a co-kernel and on top of
PREEMPT_RT in the upcoming 3.0 branch.
Will never be merged in the mainline kernel.
http://www.xenomai.org/
59
Xenomai architecture
Adeos I-Pipe
Xenomai RTOS
(nucleus)
VxWorks application
glibc
Xenomai
libvxworks

POSIX application
glibc
Xenomai
libpthread_rt
Linux application
glibc
VFS Network
Memory ...
System calls
Linux
kernel space
Pieces added
by Xenomai
Xenomai
skins
60
The Adeos interrupt pipeline abstraction
From Adeos point of view, guest OSes are prioritized domains.

For each event (interrupts, exceptions, syscalls, etc...), the
various domains may handle the event or pass it down the
pipeline.
61
Adeos virtualized interrupts disabling
Each domain may be “stalled”, meaning that it does not accept
interrupts.
Hardware interrupts
are not disabled
however (except
for the domain
leading the pipeline),
instead the interrupts
received during that
time are logged and
replayed when the
domain is unstalled.
62
Adeos additional features
The Adeos I-pipe patch implement additional features, essential
for the implementation of the Xenomai real-time extension:

Disables on-demand mapping of kernel-space vmalloc/ioremap
areas.
Disables copy-on-write when real-time processes are forking.
Allow subscribing to event allowing to follow progress of the Li
nux
kernel, such as Linux system calls, context switches, process
destructions, POSIX signals, FPU faults.
On the ARM architectures, integrates the FCSE patch, which all
ows
to reduce the latency induced by cache flushes during context
switches.
63
Xenomai features
Factored real-time core with skins implementing various real-ti
me
APIs
Seamless support for hard real-time in user-space
No second-class citizen, all ports are equivalent feature-wise
Xenomai support is as much as possible independent from the
Linux kernel version (backward and forward compatible when
reasonable)

Each Xenomai branch has a stable user/kernel ABI
Timer system based on hardware high-resolution timers
Per-skin time base which may be periodic
RTDM skin allowing to write real-time drivers
64
Xenomai user-space real-time support.
Xenomai supports real-time in user-space on 5 architectures,
including 32 and 64 bits variants.
Two modes are defined for a thread
the primary mode, where the thread is handled by Xenomai
scheduler
the secondary mode, when it is handled by Linux scheduler.
Thanks to the services of the Adeos I-pipe service, Xenomai
system calls are defined.
A thread migrates from secondary mode to primary mode when
such a system call is issued
It migrates from primary mode to secondary mode when a Linux
system call is issued, or to handle gracefully exceptional events
such as exceptions or Linux signals.

65
Life of a Xenomai application
Xenomai applications are started like normal Linux processes,
they are initially handled by the Linux scheduler and have acces
s
to all Linux services
After their initialization, they declare themselves as real-time
application, which migrates them to primary mode. In this mode
:
They are scheduled directly by the Xenomai scheduler, so they
have the real-time properties offered by Xenomai
They don't have access to any Linux service, otherwise they get
migrated back to secondary mode and looses all real-time
properties
They can only use device drivers that are implemented in Xeno
mai,
not the ones of the Linux kernel
Need to implement device drivers in Xenomai, and to split real-
time and non real-time parts of your applications.
66

Real Time Driver Model (RTDM)
An approach to unify the interfaces for developing device driver
s
and associated applications under real-time Linux
An API very similar to the native Linux kernel driver API
Allows the development, in kernel space, of
Character-style device drivers
Network-style device drivers
See the whitepaper on
http://www.xenomai.org/documentation/xenomai-2.4/pdf/RTDM
-and-Applications.pdf
Current notable RTDM based drivers:
Serial port controllers;
RTnet UDP/IP stack;
RT socket CAN, drivers for CAN controllers;
Analogy, fork of the Comedy project, drivers for acquisition car
ds.
http://www.xenomai.org/documentation/xenomai-
2.4/pdf/RTDM-and-Applications.pdf
67

Setting up Xenomai
68
How to build Xenomai
Download Xenomai sources at
http://download.gna.org/xenomai/stable/
Download one of the Linux versions supported by this release
(see ksrc/arch/<arch>/patches/)
Since version 2.0, split kernel/user building model.
Kernel uses a script called script/prepare-kernel.sh which
integrates Xenomai kernel-space support in the Linux sources.
Run the kernel configuration menu.
http://download.gna.org/xenomai/stable/
69
Linux options for Xenomai configuration

70
Xenomai user-space support
User-space libraries are compiled using the traditional autotools
./configure --target=arm-linux && make &&
make DESTDIR=/your/rootfs/ install
The xeno-config script, installed when installing Xenomai user-
space support helps you compiling your own programs.
See Xenomai's examples directory.
Installation details may be found in the README.INSTALL gui
de.
For an introduction on programming with the native API, see:
http://www.xenomai.org/documentation/branches/v2.3.x/pdf/Nat
ive-API-Tour-rev-C.pdf
For an introduction on programming with the POSIX API, see:
http://www.xenomai.org/index.php/Porting_POSIX_applications
_to_Xenomai
http://www.xenomai.org/documentation/branches/v2.3.x/pdf/Nat
ive-API-Tour-rev-C.pdf
http://www.xenomai.org/index.php/Porting_POSIX_applications
_to_Xenomai
71

Developing applications on Xenomai
72
The POSIX skin
The POSIX skin allows to recompile without changes a tradition
al
POSIX application so that instead of using Linux real-time
services, it uses Xenomai services
Clocks and timers, condition variables, message queues, mutexe
s,
semaphores, shared memory, signals, thread management
Good for existing code or programmers familiar with the POSIX
API
Of course, if the application uses any Linux service that isn't
available in Xenomai, it will switch back to secondary mode
To link an application against the POSIX skin
DESTDIR=/path/to/xenomai/
export DESTDIR
CFL=`$DESTDIR/bin/xeno-config --posix-cflags`
LDF=`$DESTDIR/bin/xeno-config --posix-ldflags`
ARCH-gcc $CFL -o rttest rttest.c $LDF

73
Communication with a normal task
If a Xenomai real-time application using the POSIX skin wishes
to
communicate with a separate non-real-time application, it must
use the rtipc mechanism
In the Xenomai application, create an IPCPROTO_XDDP socket
socket(AF_RTIPC, SOCK_DGRAM, IPCPROTO_XDDP);
setsockopt(s, SOL_RTIPC, XDDP_SETLOCALPOOL,&poolsz,
sizeof(poolsz));
memset(&saddr, 0, sizeof(saddr));
saddr.sipc_family = AF_RTIPC;
saddr.sipc_port = MYAPPIDENTIFIER;
ret = bind(s, (struct sockaddr *)&saddr, sizeof(saddr));
And then the normal socket API sendto() / recvfrom()
In the Linux application
Open /dev/rtpX, where X is the XDDP port
Use read() and write()
74
The native API (1)

A Xenomai-specific API for developing real-time tasks
Usable both in user-space and kernel space. Development of tas
ks
in user-space is the preferred way.
More coherent and more flexible API than the POSIX API. Easi
er to
learn and understand. Certainly the way to go for new applicatio
ns.
Applications should include <native/service.h>, where
service can be alarm, buffer, cond, event, heap,
intr, misc, mutex, pipe, queue, sem, task, timer
To compile applications :
DESTDIR=/path/to/xenomai/
export DESTDIR
CFL=`$DESTDIR/bin/xeno-config --xeno-cflags`
LDF=`$DESTDIR/bin/xeno-config --xeno-ldflags`
ARCH-gcc $CFL -o rttest rttest.c $LDF -lnative
75
The native API (2)
Task management services
rt_task_create(), rt_task_start(),
rt_task_suspend(), rt_task_resume(),
rt_task_delete(), rt_task_join(), etc.

Counting semaphore services
rt_sem_create(), rt_sem_delete(), rt_sem_p(),
rt_sem_v(), etc.
Message queue services
rt_queue_create(), rt_queue_delete(),
rt_queue_alloc(), rt_queue_free(),
rt_queue_send(), rt_queue_receive(), etc.
Mutex services
rt_mutex_create(), rt_mutex_delete(),
rt_mutex_acquire(), rt_mutex_release(), etc.
76
The native API (3)
Alarm services
rt_alarm_create(), rt_alarm_delete(),
rt_alarm_start(), rt_alarm_stop(),
rt_alarm_wait(), etc.
Memory heap services
Allows to share memory between processes and/or to pre-allocat
e
a pool of memory

rt_heap_create(), rt_heap_delete(),
rt_heap_alloc(), rt_heap_bind()
Condition variable services
rt_cond_create(), rt_cond_delete(),
rt_cond_signal(), rt_cond_broadcast(),
rt_cond_wait(), etc.
77
Xenomai and normal task communication
Using rt_pipes
In the native Xenomai application, use the Pipe API
rt_pipe_create(), rt_pipe_delete(),
rt_pipe_receive(), rt_pipe_send(),
rt_pipe_alloc(), rt_pipe_free()
In the normal Linux application
Open the corresponding /dev/rtpX file, the minor is specified at
rt_pipe_create() time
Then, just read() and write() to the opened file
Xenomai application
Uses the rt_pipe_*() API
Linux application

open(“/dev/rtpX”)
78
Real-time approaches
The following table is Paul Mac Kenney's summary of his own
article describing the various approaches for real-time on Linux
:
Inspection API
POSIX + RT N/A None
PREEMPT POSIX + RT N/A None
RTOS Excellent
PREEMPT_RT POSIX + RT None
OK
POSIX + RT None
Approach Quality Complexity
Fault
isolation
HW/SW
Configs
Vanilla Linux

10s of ms
all services All All
100s of us
Schd, Int
preempt or
irq disable All
Nested OS
(co-kernel)
~10us
RTOS svcs
RTOS,
hw irq disable
RTOS (can
be POSIX RT) Dual env. Good All
Dual-OS/Dual-Core
(ASMP)
<1us
RTOS svcs
RTOS (can
be POSIX RT) Dual env. Specialized
10s of us
Schd, Int
preempt and irq
disable (most
ints in process ctx),
(mostly drivers)

"Modest" patch
(careful tuning)
All (except
some
drivers)
Migration between OSes ? us
RTOS svcs
RTOS,
hw irq disable
RTOS (can
be POSIX RT)
Dual env. (easy
mix) All
Migration within OS
? us
RTOS svcs
Sched,
RTOS svcs Small patch All?
(additions in blue)
Full story at http://lwn.net/Articles/143323
http://lwn.net/Articles/143323
79

Books
Building Embedded Linux Systems, O'Reilly
By Karim Yaghmour, Jon Masters,
Gilad Ben-Yossef, Philippe Gerum and others
(including Michael Opdenacker), August 2008
A nice coverage of Xenomai (Philippe Gerum)
and the RT patch (Steven Rostedt)
http://oreilly.com/catalog/9780596529680/
http://oreilly.com/catalog/9780596529680/
80
Organizations
http://www.realtimelinuxfoundation.org/
Community portal for real-time Linux.
Organizes a yearly workshop.
http://www.osadl.org
Open Source Automation Development Lab (OSADL)
Created as an equivalent of OSDL for machine and plant control
systems. Member companies are German so far (Thomas Gleixn
er
is on board). One of their goals is to supports the development o
f
RT preempt patches in the mainline Linux kernel (HOWTOs, liv
e
CD, patches).

http://www.realtimelinuxfoundation.org/
http://www.osadl.org/
nt, consulting, training and support. http//free-electrons.com
Related documents
All our technical presentations
on http://free-electrons.com/docs
Linux kernel
Device drivers
Architecture specifics
Embedded Linux system development
http://free-electrons.com/docs
nt, consulting, training and support. http//free-electrons.com
How to help
You can help us to improve and maintain this document...
By sending corrections, suggestions, contributions and
translations
By asking your organization to order development, consulting
and training services performed by the authors of these
documents (see http://free-electrons.com/).
By sharing this document with your friends, colleagues

and with the local Free Software community.
By adding links on your website to our on-line materials,
to increase their visibility in search engine results.
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Kernel debugging
Question 1 Real time issues in embedded systems
Write a discussion of the issues involved in designing real time
systems. Include the following systems: Bare metal
programming, Linux or Unix operating systems, and
microkernel designs.
Describe the advantages and potential problems of these types
of system
Some information and references can be found in the folder
Question 2 JTAG description, and use for testing and flash
programming
To answer this question, you may cut and paste any diagrams
and paraphrase, but preferably condense, any text you wish.
Please reference all sources you use. Format your answer as you
wish, but try to cover all the following points.
Using the references below and/or any others you can find,
answer the following questions.
Describe briefly the principles and methods of the boundary
scan testing through the JTAG interface. Refer to:
http://en.wikipedia.org/wiki/Joint_Test_Action_Group

And
http://boundaryscan.blogspot.ca/2010/04/tutorial-role-of-jtag-
in-system-debug.html
and
http://www.corelis.com/education/JTAG_Tutorial.htm

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