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

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Message-ID: <1307717917.9218.45.camel@gandalf.stny.rr.com>
Date:	Fri, 10 Jun 2011 10:58:37 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Hillf Danton <dhillf@...il.com>
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, 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.

-- Steve


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