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Linux Kernel Process Management

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What to Expect?
W's of Threads & Kthreads
W's of Concurrency Management
Waiting / Sleeping in a process
'select & poll' support in kernel

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W's of Threads
Basic unit of CPU utilization
Also, known as Light weight process
Implemented as POSIX threads in user space,
commonly known as pthread
Multiple threads share the same address space in
a process
Communication is usually through global variables

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W's of Kernel Threads
Exist in Kernel Space
Used to perform some tasks in background
Are descended from kthreadd
Can be viewed in user space with command
'ps -fx' or, 'ps -mx'
Examples:
khubd
ksoftirqd
kworker

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Creating a Kernel Thread
#include <kthread.h>
kthread_create(int (*function)(void *data), void *data, const char
name[], ...)
Parameters:
function – The function that the thread has to execute
data – The 'data' to be passed to the function
name – The name by which the process will be recognized in the kernel
Returns:
Pointer to a structure of type task_struct
Try Threads/thread.c

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Waking Up & Stopping a Thread
wake_up_process(struct task_struct *)
Function passed to kthread_create() gets executed
Parameters:
Pointer to struct of type task_struct
kthread_stop(struct task_struct *)
Waits for the thread to stop
Parameters
Pointer to structure of type task_struct
Returns:
Result of thread function
Try Threads/thread1.c and Threads/thread_stop.c

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Running a Thread
kthread_run(int (*function)(void *data), void *data,
const char name[], ...)
Creates & starts the threads
Parameters:
function – The function that the thread has to execute
data – The 'data' to be passed to the function
name – The name by which the process will be recognized in the
kernel
Try Threads/thread_run.c

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Concurrency Management

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Concurrency Issues
Multitasking leads to the uncontrolled access to
the shared resources
Easiest to create & hard to debug
May lead to memory leak
Global variable should best be avoided
If can't avoid global variables, then need to
resort to the synchronization mechanisms

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Mutex
Stands for MUTual EXclusion
Only one thread of execution can hold it at a
time.
Has operations such as lock & unlock.
'lock' call blocks if mutex is not available & the
process goes to sleep

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Data Structures for Mutex
#include <linux/mutex.h>
struct mutex
Initialization
Statically
DEFINE_MUTEX(mutex)
Dynamically
mutex_init
Operations
mutex_lock()
mutex_unlock()
mutex_lock_interruptible()
mutex_trylock()

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Hands On
Try Synchronization/mutex.c and
Synchronization/mutex1.c

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Semaphore
Semaphore is a counter, which keeps track of
resources
Has two operations – wait/down & post/up
Wait decrements the count by one
Post increments the count by one
Process blocks if the value of count is zero.

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Data Structures for Semaphore
#include <linux/semaphore.h>
struct semaphore
Initialization
Statically
DEFINE_SEMAPHORE(name)
Dynamically
sema_init(&sem, val)
Operations
void down((&sem)
int down_interruptible(&sem)
Int down_trylock()
void up(&sem)

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Hands On
Try Synchronization/sem.c
Try Synchronization/cons_prod.c
Modify cons_prod.c to avoid exit of consumers
threads

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Reader/Writer Semaphores
#include <linux/rwsem.h>
structure rw_semaphore
Allows one writer or unlimited number of readers to hold the
semaphore
Intialization:
void init_rwsem(&rwsem)
Operations for Readers
void down_read(&sem)
int down_read_trylock(&sem)
void up_read(&sem)

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Reader/Writer Semaphores...
Operations for Writers
void down_write(struct rw_semaphore *sem)
int down_write_trylock(struct rw_semaphore *sem)
void up_write(struct rw_semaphore *sem)
Try Synchronization/rwsem.c

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Spinlocks
Provides synchronization mechanism for the code
which can't sleep. Example – Interrupt handlers
Two states – locked & unlocked
If lock is available, code enters the critical section
If lock is unavailable, code goes into the tight loop.
So, the name spinlock

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Spinlock Data Structures
#include <linux/spinlock.h>
Type: spinlock_t
spin_lock_init(&my_slock)
spin_lock(&my_slock)
spin_unlock(&my_slock)
Try spinlock_example.c

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Cautions while holding a spin lock
Avoid the code which can sleep
Preemption is disabled by spin lock code
Critical section should be fast & short
Sometimes, disabling of interrupts on the local
processor is required to avoid deadlock

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Other spinlock functions
spin_lock_irqsave(&my_lock, flags)
Disables interrupts on the local processor
Interrupt state is saved in flags
spin_lock_irq(&my_slock)
Disables interrupts, but doesn't keeps track of flags
spin_lock_bh
Disables software interrupts only

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spin_unlock variants
void spin_unlock_irqrestore(&my_slock, flags)
Unlocks & restores the irqs
void spin_unlock_irq(&my_slock)
void spin_unlock_bh(&my_slock)
Try Synchronization/spinlock.c and
Synchronization/spinlock1.c

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Kernel thread states
TASK_RUNNING: Thread is in run queue or is running
TASK_INTERRUPTIBLE: Waiting for an event to occur. Wakes on event or
signal and moves to run queue
TASK_UINTERRUPTIBLE: Similar to TASK_INTERRUPTIBLE, receipt of a
signal has not effect
TASK_TRACED: If strace is used to trace the thread
EXIT_ZOMBIE: Terminated, but not cleaned up yet

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Waiting / Sleeping in a Process
Task moves out of run queue and waits for an event.
Also, known as sleeping
Event can be:
Specified amount of time
Data from file IO
Waiting on semaphore or mutex
Task can be in TASK_INTERRUPTIBLE or
TASK_UNITERRUPTIBLE state

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Schedule() & set_current_state()
schedule()
Used by processes to voluntarily relinquish the CPU
set_current_state(state)
Sets the state of the process to:
TASK_INTERRUPTIBLE
TASK_UNITERRUPTIBLE
TASK_RUNNING

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wake_up_process
Wakes up the process & changes the process
state to TASK_RUNNING
wake_up_process(struct task_struct *);

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Example Codes
Try WaitQueues/sched.c &
WaitQueues/sched1.c
Modify sched1.c to wait for an event

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Wait Queues
Set of sleeping processes waiting for an event
#include <linux/wait.h>
Data structure
wait_queue_head_t
Created statically
DECLARE_WAITQUEUE(wait_queue_name)
Created dynamically
wait_queue_head_t my_queue
init_waitqueue_head(&my_queue)

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Variants of wait
wait_event(queue, condition)
wait_event_interruptible(queue, condition)
wait_event_timeout(queue, condition, timeout)
wait_event_interruptible_timeout(queue,
condition, timeout)

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Waking up the process
wake_up family of functions
Wakes up all the processes waiting on a queue
wake_up(wake_queue_head_t *)
Wakes up all the processes waiting on the queue
wake_up_interruptible(wait_queue_head_t *)
Wakes up only the processes performing the
interruptible sleep

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Hands On
WaitQueues/wait_queue.c
Modify an example WaitQueues/sched2.c to
use the wait queues

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Waiting using wait queue entry
Initialize of wait queue entry
Statically
DEFINE_WAIT(wait_queue)
Dynamically
wait_queue_t wait_queue
init_wait(&wait_queue)
Add an entry to the queue
void prepare_to_wait()
Parameters:
Pointer to wait_queue_head_t
Pointer to wait_queue_t
State
Cleanup:
Void finish_wait()
Pointer to wait_queue_head_t
Pointer to wait_queue_t

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Hands On
Try WaitQueues/mwait.c
Modify mwait.c to wait for an event

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Thundering Herd Problem
'wake_up' awakens all the processes waiting on
a wait queue
Only one will succeed in obtaining the desired
resource & rest will have to sleep
For large number of processes on wait queue,
this 'thundering herd' may degrade the system
performance

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Exclusive wait
Exclusive waiters get added to the end of the
wait queue
wake_up on the wait queue stops after
awakening the first exclusive waiter
void prepare_to_wait_exclusive() sets the
'exclusive' flag in the wait queue
Can't be performed wait_event()

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Hands On
Try WaitQueues/wait_thunder.c
Modify above example to prevent thundering
herd

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wake_up variants
wake_up()
wake_up_interruptible()
wake_up_nr()
wake_up_interruptible_nr()
wake_up_all()
wake_up_interruptible_all()

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poll and select
Allows an application to wait on more than one inputs or
outputs
Application blocks, if no data is available
Example, hyperterminal/minicom needs to wait on user input
as well as serial data
Multiple system calls for same functionality
poll – System V solution
select – BSD unix
epoll – for supporting thousand's of file descriptors

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Driver support for select & poll
#include <linux/poll.h>
Support in driver is provided through poll method
unsigned int (*poll) (struct file *filp, poll_table *wait)
Does the following
Adds the wait_queues to poll_table that can indicate the change in the poll status.
void poll_wait(struct file *, wait_queue_head_t *, poll_table *)
Returns a bit mask for the operations that can be performed without blocking
POLLIN
POLLRDNORM
POLLHUP
POLLOUT
POLLWRNORM

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Hands On
Select/select.c – Driver
Apps/select_ap – Application

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What all have learnt?
W's of Threads & Kthreads
W's of Concurrency Management
Waiting / Sleeping in a Process
'select & poll' support in Kernel

42
© 2014-2015 SysPlay Workshops <workshop@sysplay.in>
All Rights Reserved.
Any Queries?