linux-kernel - Re: [PATCH 2/2] sched: Update runtime of prev task before doing pick_next

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Message-ID: <xm26oal7s00b.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Tue, 26 May 2015 10:48:36 -0700
From:	bsegall@...gle.com
To:	Kirill Tkhai <ktkhai@...n.com>
Cc:	<linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Mohammed Naser <mnaser@...xhost.com>,
	"Ingo Molnar" <mingo@...nel.org>, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH 2/2] sched: Update runtime of prev task before doing pick_next_task()

Kirill Tkhai <ktkhai@...n.com> writes:

> pick_next_task() puts prev rq's task. This may lead to runtime
> expiration and to dequeueing of all scheduling class's tasks
> because of throttling. And the current logic is that put_prev_task()
> must be called in the pick method of next task's class.
>
> This was fixed for RT and DL classes, while fair class have this
> problem. So, instead of doing partial solutions, let's update prev
> task's runtime for all classes in __schedule() and fix the problem
> completelly.
>
> Also, let's freeze the clock during pick_next_task() to be sure
> new expirations of runtime won't happen.
>
> Reported-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Reported-by: Mohammed Naser <mnaser@...xhost.com>
> Signed-off-by: Kirill Tkhai <ktkhai@...n.com>


If this is actually the bug I sent a patch for (and that I was correct
in guessing what the issue /was/, which is not at all certain), this
won't actually eliminate the issue - I couldn't find a race involving
actual updates but did find one with disable/enable.
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