linux-kernel - Re: [PATCH v2] sched: fix clear NOHZ_BALANCE

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Date:	Wed, 5 Jun 2013 12:44:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	mingo@...nel.org, fweisbec@...il.com
Subject: Re: [PATCH v2] sched: fix clear NOHZ_BALANCE_KICK

On Wed, Jun 05, 2013 at 10:13:11AM +0200, Vincent Guittot wrote:
> I have faced a sequence where the Idle Load Balance was sometime not
> triggered for a while on my platform.
> 
> CPU 0 and CPU 1 are running tasks and CPU 2 is idle
> 
> CPU 1 kicks the Idle Load Balance
> CPU 1 selects CPU 2 as the new Idle Load Balancer
> CPU 2 sets NOHZ_BALANCE_KICK for CPU 2
> CPU 2 sends a reschedule IPI to CPU 2
> While CPU 3 wakes up, CPU 0 or CPU 1 migrates a waking up task A on CPU 2
> CPU 2 finally wakes up, runs task A and discards the Idle Load Balance
> task A quickly goes back to sleep (before a tick occurs on CPU 2)
> CPU 2 goes back to idle with NOHZ_BALANCE_KICK set
> 
> Whenever CPU 2 will be selected as the ILB, no reschedule IPI will be sent
> because NOHZ_BALANCE_KICK is already set and no Idle Load Balance will be
> performed.
> 
> We must wait for the sched softirq to be raised on CPU 2 thanks to another
> part the kernel to come back to clear NOHZ_BALANCE_KICK.
> 
> The proposed solution clears NOHZ_BALANCE_KICK in schedule_ipi if
> we can't raise the sched_softirq for the Idle Load Balance.
> 
> Change since V1:
> - move the clear of NOHZ_BALANCE_KICK in got_nohz_idle_kick if the ILB
> can't run on this CPU (as suggested by Peter)
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks!
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