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

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Date:	Tue, 11 Feb 2014 22:21:38 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...capital.net>
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, 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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