linux-kernel - Re: [PATCH 4/6] sched,idle: Clear polling before descheduling the idle thread

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Date:	Wed, 4 Jun 2014 07:46:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, nicolas.pitre@...aro.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] sched,idle: Clear polling before descheduling the
 idle thread

On Wed, Jun 4, 2014 at 12:53 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jun 03, 2014 at 05:29:50PM -0700, Andy Lutomirski wrote:
>> @@ -229,6 +234,8 @@ static void cpu_idle_loop(void)
>>                */
>>               preempt_set_need_resched();
>>               tick_nohz_idle_exit();
>> +             __current_clr_polling();
>> +             smp_mb__after_clear_bit();
>
> barriers always need a comment, and I'm not entirely sure why you put
> this one here.

Will do.

I actually suspect that the barrier isn't needed in this patch, but it
will be needed one patch later when there's a sched_ttwu_pending after
the barrier.  I'll move this to the next patch and add a comment.

The reason that the barrier is needed is that polling needs to be
visibly cleared before checking for queued ttwu tasks, because
otherwise another CPU could queue a ttwu task, do set_nr_if_polling,
and get a true return value *after* sched_ttwu_pending because the CPU
decided to defer the clear_bit.

--Andy
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