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Date:   Tue, 17 Mar 2020 20:24:01 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Josh Don <joshdon@google.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Li Zefan <lizefan@huawei.com>, Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Paul Turner <pjt@google.com>
Subject: Re: [PATCH v2] sched/cpuset: distribute tasks within affinity masks
Message-ID: <20200317192401.GE20713@hirez.programming.kicks-ass.net>
References: <20200311010113.136465-1-joshdon@google.com>
 <20200311140533.pclgecwhbpqzyrks@e107158-lin.cambridge.arm.com>
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On Wed, Mar 11, 2020 at 02:05:33PM +0000, Qais Yousef wrote:
> On 03/10/20 18:01, Josh Don wrote:
> > From: Paul Turner <pjt@google.com>
> > 
> > Currently, when updating the affinity of tasks via either cpusets.cpus,
> > or, sched_setaffinity(); tasks not currently running within the newly
> > specified mask will be arbitrarily assigned to the first CPU within the
> > mask.
> > 
> > This (particularly in the case that we are restricting masks) can
> > result in many tasks being assigned to the first CPUs of their new
> > masks.
> > 
> > This:
> >  1) Can induce scheduling delays while the load-balancer has a chance to
> >     spread them between their new CPUs.
> >  2) Can antogonize a poor load-balancer behavior where it has a
> >     difficult time recognizing that a cross-socket imbalance has been
> >     forced by an affinity mask.
> > 
> > This change adds a new cpumask interface to allow iterated calls to
> > distribute within the intersection of the provided masks.
> > 
> > The cases that this mainly affects are:
> > - modifying cpuset.cpus
> > - when tasks join a cpuset
> > - when modifying a task's affinity via sched_setaffinity(2)
> > 
> > Co-developed-by: Josh Don <joshdon@google.com>
> > Signed-off-by: Josh Don <joshdon@google.com>
> > Signed-off-by: Paul Turner <pjt@google.com>

> Anyway, for the API.
> 
> Reviewed-by: Qais Yousef <qais.yousef@arm.com>
> Tested-by: Qais Yousef <qais.yousef@arm.com>

Thanks guys!