scheduler context

[Arun KS]

Hi Ratheesh,

I ll give a try.

On Wed, Apr 17, 2013 at 9:43 PM, ratheesh kannoth <ratheesh.ksz at 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
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20130418/a15816e4/attachment.html