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

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Message-ID: <54212B85.7060806@arm.com>
Date:	Tue, 23 Sep 2014 09:12:53 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"juri.lelli@...il.com" <juri.lelli@...il.com>,
	"raistlin@...ux.it" <raistlin@...ux.it>,
	"michael@...rulasolutions.com" <michael@...rulasolutions.com>,
	"fchecconi@...il.com" <fchecconi@...il.com>,
	"daniel.wagner@...-carit.de" <daniel.wagner@...-carit.de>,
	"vincent@...out.info" <vincent@...out.info>,
	"luca.abeni@...tn.it" <luca.abeni@...tn.it>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Li Zefan <lizefan@...wei.com>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating
 tasks between exclusive cpusets

Hi Peter,

On 19/09/14 22:25, Peter Zijlstra wrote:
> 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.
> 

Right, next week I should be able to properly test this.

Thanks a lot,

- Juri

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