linux-kernel - Re: [PATCH 3/5] sched/deadline: Fix wrong cpudl_find() in check_preempt_equal

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Date:	Wed, 28 Jan 2015 23:18:34 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH 3/5] sched/deadline: Fix wrong cpudl_find() in check_preempt_equal_dl()

Hi Peter,

On 28 January 2015 at 00:47, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Jan 19, 2015 at 04:49:38AM +0000, Xunlei Pang wrote:
>> In check_preempt_equal_dl(), cpudl_find() is called with a NULL
>> later_mask, thus cpudl_find() here doesn't check cpudl::free_cpus
>> at all.
>>
>> This patch takles this issue by always passing a non-NULL later_mask
>> to cpudl_find(), thereby fixing this issue.
>
> Fix what issue? Afaict this is an optimization not a fix.

Currently, check_preempt_equal_dl() invokes cpudl_find() with a NULL mask,
so cpudl_find() won't check cpudl::free_cpus. For example, 4 core
system, CPU0~CPU2
are all idle(free of deadline tasks), a deadline task is woken on CPU3
which already
has one running deadline task with the same deadline value, then
cpudl_find() will fail
causing CPU3 holding 2 deadline tasks while other cpus are idle,
obviously it should
be placed on one idle cpu.

Thanks,
Xunlei
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