linux-kernel - Re: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating tasks between exclusive cpusets

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Date:	Fri, 19 Sep 2014 13:47:34 +0200
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	Juri Lelli <juri.lelli@....com>, <peterz@...radead.org>
CC:	<mingo@...hat.com>, <juri.lelli@...il.com>, <raistlin@...ux.it>,
	<michael@...rulasolutions.com>, <fchecconi@...il.com>,
	<vincent@...out.info>, <luca.abeni@...tn.it>,
	<linux-kernel@...r.kernel.org>, Li Zefan <lizefan@...wei.com>,
	<cgroups@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating
 tasks between exclusive cpusets

Hi,

On 09/19/2014 11:22 AM, Juri Lelli wrote:
> Exclusive cpusets are the only way users can restrict SCHED_DEADLINE tasks
> affinity (performing what is commonly called clustered scheduling).
> Unfortunately, such thing is currently broken for two reasons:
> 
>  - No check is performed when the user tries to attach a task to
>    an exlusive cpuset (recall that exclusive cpusets have an
>    associated maximum allowed bandwidth).
> 
>  - Bandwidths of source and destination cpusets are not correctly
>    updated after a task is migrated between them.
> 
> This patch fixes both things at once, as they are opposite faces
> of the same coin.
> 
> The check is performed in cpuset_can_attach(), as there aren't any
> points of failure after that function. The updated is split in two
> halves. We first reserve bandwidth in the destination cpuset, after
> we pass the check in cpuset_can_attach(). And we then release
> bandwidth from the source cpuset when the task's affinity is
> actually changed. Even if there can be time windows when sched_setattr()
> may erroneously fail in the source cpuset, we are fine with it, as
> we can't perfom an atomic update of both cpusets at once.
> 
> Reported-by: Daniel Wagner <daniel.wagner@...-carit.de>

Ack

I have no special test for this, I just let my test running which was
fixed by patch #1. Works fine though. I'll plan to write some test for this.

Thanks,
Daniel
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