Details about Softirq, Tasklet and workqueue in Linux kernel. Difference between all these. All activation finction such as initialization, activation and mask.

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Bottom Half in Linux Kernel
(LK)

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Bottom half
 Execute in Interrupt context after interrupt
handler (often called Top Half) complete


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Why Bottom half
 Interrupt potentially other important code
 High priory interrupt stop execution of low priority
 Worst case of IRQ_DISABLED disable all
interrupt
 Interrupt are often time critical
 Can’t be block as it is in interrupt context

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Type of Bottom half in LK 2.6
 Softirqs
 Tasklets
 Work queues

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Softirq
 Created at compile time
 Of same/different type can run
simultaneously on any
processor
 For performance critical such
networking
 Softirq doesn’t preempt other
Softirq
 Tasklets and kernel
timers built on top of
Softirq

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Softirq → Index (priority) → Description
HI_SOFTIRQ 0 Handles high priority tasklets
TIMER_SOFTIRQ 1 Tasklets related to timer interrupts
NET_TX_SOFTIRQ 2 Transmits packets to network cards
NET_RX_SOFTIRQ 3 Receives packets from network cards
SCSI_SOFTIRQ 4 Post-interrupt processing of SCSI
commands
TASKLET_SOFTIRQ 5 Handles regular tasklets

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Softirq
 Initialization
 open_softirq()
 Activation
 raise_softirq( )
 Masking
 __local_bh_disable( )/__local_bh_enable( )
 Execution
 void softirq_handler(struct softirq_action *)

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Tasklet
 Built on top of Softirqs i.e. HI_SOFTIRQ and
TASKLET_SOFTIRQ
 HI_SOFTIRQ Tasklet runs prior to TASKLET_SOFTIRQ

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Tasklet
Initialization
tasklet_init( )
Activation
tasklet_schedule( ) or asklet_hi_schedule( )
Masking
tasklet_disable( )/tasklet_enable( )
Execution
tasklet_action( )/tasklet_hi_action( )

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Softirq <-----------> Tasklet
 Only for high frequency and
highly threaded uses
 Created statically
 Simultaneous execution of
same Softirq
 For all other cases where
interrupt context is needed
 Can be created statically or
dynamically
 Two of same Tasklet can’ get
executed but two different
Tasklet can run on different
processor
 Built on top of Softirq

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Work Queues
 Defers work in kernel thread also called
worker thread
 Run in process context
 Used in case if deffered work can sleep

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Workqueue implementation
 Initialization
 create_workqueue() - creates n worker threads (n - no of
CPU)
 Activation
 queue_work( )
 Masking

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Tasklet/Softirq ←-- → Work queue
 Used when
deferred work can’t
sleep
 Not schedulable
 Used when
deferred work can
sleep
 Schedulable

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