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

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Message-ID: <20140919212547.GG2832@worktop.localdomain>
Date:	Fri, 19 Sep 2014 23:25:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	mingo@...hat.com, juri.lelli@...il.com, raistlin@...ux.it,
	michael@...rulasolutions.com, fchecconi@...il.com,
	daniel.wagner@...-carit.de, 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

On Fri, Sep 19, 2014 at 10:22:40AM +0100, 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.

The thing I cannot find is if we correctly deal with updates to the
cpuset. Say we first setup 2 (exclusive) sets A:cpu0 B:cpu1-3. Then
assign tasks and then update the cpu masks like: B:cpu2,3, A:cpu1,2.


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