linux-kernel - Re: [PATCH 00/18] sched: balance callbacks v4

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Date:	Tue, 29 Dec 2015 14:41:30 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	umgwanakikbuti@...il.com, mingo@...e.hu, ktkhai@...allels.com,
	rostedt@...dmis.org, tglx@...utronix.de, juri.lelli@...il.com,
	pang.xunlei@...aro.org, oleg@...hat.com,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/18] sched: balance callbacks v4

On Thu, Jun 11, 2015 at 02:46:36PM +0200, Peter Zijlstra wrote:
> Mike stumbled over a cute bug where the RT/DL balancing ops caused a bug.

Hello.

We also faced this bug in our embedded product using stable tree. Eventually,
we found that these patch set exactly dealt with the problem. Now, I am just
curious about if you have any plans to merge this to stable trees to make
the stable trees stabler.

Hm?

> 
> The exact scenario is __sched_setscheduler() changing a (runnable) task from
> FIFO to OTHER. In swiched_from_rt(), where we do pull_rt_task() we temporarity
> drop rq->lock. This gap allows regular cfs load-balancing to step in and
> migrate our task.
> 
> However, check_class_changed() will happily continue with switched_to_fair()
> which assumes our task is still on the old rq and makes the kernel go boom.
> 
> Instead of trying to patch this up and make things complicated; simply disallow
> these methods to drop rq->lock and extend the current post_schedule stuff into
> a balancing callback list, and use that.
> 
> This survives Mike's testcase.
> 
> Changes since -v3:
>  - reworked the hrtimer stuff, again. -- Kirill, Oleg
>  - small changes to the new lockdep stuff
> 
> Changes since -v2:
>  - reworked the hrtimer patch. -- Kirill, tglx
>  - added lock pinning
> 
> Changes since -v1:
>  - make SMP=n build,
>  - cured switched_from_dl()'s cancel_dl_timer().
> 
> no real tests on the new parts other than booting / building kernels.
> 
> 
> 
> --
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