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SubjectRe: Too many rescheduling interrupts (still!)
On Tue, 11 Feb 2014, Andy Lutomirski wrote:

Just adding Peter for now, as I'm too tired to grok the issue right
now.

> Rumor has it that Linux 3.13 was supposed to get rid of all the silly
> rescheduling interrupts. It doesn't, although it does seem to have
> improved the situation.
>
> A small number of reschedule interrupts appear to be due to a race:
> both resched_task and wake_up_idle_cpu do, essentially:
>
> set_tsk_need_resched(t);
> smb_mb();
> if (!tsk_is_polling(t))
> smp_send_reschedule(cpu);
>
> The problem is that set_tsk_need_resched wakes the CPU and, if the CPU
> is too quick (which isn't surprising if it was in C0 or C1), then it
> could *clear* TS_POLLING before tsk_is_polling is read.
>
> Is there a good reason that TIF_NEED_RESCHED is in thread->flags and
> TS_POLLING is in thread->status? Couldn't both of these be in the
> same field in something like struct rq? That would allow a real
> atomic op here.
>
> The more serious issue is that AFAICS default_wake_function is
> completely missing the polling check. It goes through
> ttwu_queue_remote, which unconditionally sends an interrupt.
>
> --Andy
>


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