linux-kernel - Re: Too many rescheduling interrupts (still!)

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Date:	Tue, 11 Feb 2014 14:34:11 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Mike Galbraith <bitbucket@...ine.de>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Too many rescheduling interrupts (still!)

On Tue, Feb 11, 2014 at 1:21 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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.

There would be an extra benefit of moving the resched-related bits to
some per-cpu structure: it would allow lockless wakeups.
ttwu_queue_remote, and probably all of the other reschedule-a-cpu
functions, could do something like:

if (...) {
  old = atomic_read(resched_flags(cpu));
  while(true) {
      if (old & RESCHED_NEED_RESCHED)
        return;
      if (!(old & RESCHED_POLLING)) {
        smp_send_reschedule(cpu);
        return;
      }
      new = old | RESCHED_NEED_RESCHED;
      old = atomic_cmpxchg(resched_flags(cpu), old, new);
  }
}

The point being that, with the current location of the flags, either
an interrupt needs to be sent or something needs to be done to prevent
rq->curr from disappearing.  (It probably doesn't matter if the
current task changes, because TS_POLLING will be clear, but what if
the task goes away entirely?)

All that being said, it looks like ttwu_queue_remote doesn't actually
work if the IPI isn't sent.  The attached patch appears to work (and
reduces total rescheduling IPIs by a large amount for my workload),
but I don't really think it's worthy of being applied...

--Andy

View attachment "0001-sched-Try-to-avoid-sending-an-IPI-in-ttwu_queue_remo.patch" of type "text/x-diff" (2127 bytes)