linux-kernel - Re: [PATCH] sched: remove resetting exec_start in put_prev_task

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Date:	Fri, 10 Jun 2011 23:12:56 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: remove resetting exec_start in put_prev_task_rt()

On Fri, Jun 10, 2011 at 10:58 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 2011-06-10 at 22:48 +0800, Hillf Danton wrote:
>> On Thu, Jun 9, 2011 at 8:34 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
>> Resetting exec_start to zero has no negative functionality in RT scheduling,
>> as shown by Yong.
>>
>> After put_prev_task() is called in schedule(),
>>
>>       put_prev_task(rq, prev);
>>       next = pick_next_task(rq);
>>       clear_tsk_need_resched(prev);
>>
>> next is picked. Lets assume that next is not prev, and prev is still on RQ,
>> then prev's sched_class is changed to CFS while it is waiting on RQ.
>> After sched_class switch, prev is under CFS charge, and the exec_start field
>> could be taken into other games.
>>
>> In task_hot(), called when migrating task, zeroed exec_start is trapped as
>> the following.
>>
>
> How could any of that happen? This is all done under the rq->lock.
> prev's sched class can not change at this time. Everything you stated is
> protected by the rq->lock. I don't see any race conditions here.
>

Hi Steve

Yeah you are right, the snippet in schedule() is under RQ lock, but next could
be a RT task, and if it is FIFO and willing to hog CPU ie. 10 minutes, then
prev has to wait on its RQ. While waiting, however, its sched_class could be
changed at anytime. Here I show it is tiny possible that zeroed exec_start
could rush out of our control.

thanks
           Hillf
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