Hi Ratheesh, I ll give a try. On Wed, Apr 17, 2013 at 9:43 PM, ratheesh kannoth <ratheesh.ksz@gmail.com>wrote:

I would like to understand on linux scheduler context. I have read a lot in websites and i could find contradictory statement. There are so many mailing list also ,but with less info. I would really appreciate if anybody could spend some time answering my question.

1. Which context scheduler run ? ( process context or interrupt context ).

Process context. That is when preempt_count equal 0

2. scheduler is the guy who picks up the next candidate to run. if it gets interrupted by hardirq, what will happen ?

scheduler resumes from where it was stopped, after finishing hardirq handler(and softirq handlers if any)

3. if scheduler run in process context , how bottom half are scheduled ?

Bottom halves(softirqs and tasklets) are not scheduled by scheduler. ISR is executed in_irq() context or with preempt count added with HARDIRQ_OFFSET. Local interrupts are disabled in_irq() context. After completing ISR, irq_exit() function checks for any pending softirqs, if yes it invokes invoke_softirq() to service all pending softirqs with interrrupts enabled. When softirqs runs, preempt count will be added with SOFTIRQ_OFFSET. 4. In smp, schedule() function may be called simultaneously. How it

is handled ?

Each core has its own runqueue which is a per_cpu variable.

5. When a bottom half is interrupted by hard irq, how softirq kernel thread saves the state and restart it later ? ...what i mean is , an hardirq came and its isr executed. bottom half enabled and bottom half gets scheduled . Before bottom half is completed , next irq came and it enables bottom half , and the new bottom half is scheduled ....So here , what will happen to old bottom half , will it again run later ?

The old bottom half handler(which was preempted by irq) will be resumed after the hardirq isr. Followed by the next bottom half. Refer function __do_softirq in kernel/softirq.c Thanks, Arun

Thanks, Ratheesh

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