linux-kernel - Re: [PATCH v2] sched/topology, cpuset: Account for housekeeping CPUs to avoid empty cpumasks

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Date:   Fri, 15 Nov 2019 18:18:07 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        lizefan@...wei.com, tj@...nel.org, hannes@...xchg.org,
        mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        Dietmar.Eggemann@....com, morten.rasmussen@....com,
        qperret@...gle.com
Subject: Re: [PATCH v2] sched/topology, cpuset: Account for housekeeping CPUs
 to avoid empty cpumasks

Hello.

On Thu, Nov 14, 2019 at 04:03:50PM +0000, Valentin Schneider <valentin.schneider@....com> wrote:
> Michal, could I nag you for a reviewed-by? I'd feel a bit more confident
> with any sort of approval from folks who actually do use cpusets.
TL;DR I played with the v5.4-rc6 _without_ this fixup and I conclude it
unnecessary (IOW my previous theoretical observation was wrong).

The original problem is non-issue with v2 cpuset controller, because
effective_cpus are never empty. isolcpus doesn't take out cpuset CPUs,
hotplug does. In the case, no online CPU remains in the cpuset, it
inherits ancestor's non-empty cpuset.

I reproduced the problem with v1 (before your fix). However, in v1
effective == allowed (we're destructive and overwrite allowed on
hotunplug) and we already check the emptiness of 

  cpumask_intersects(cp->cpus_allowed, housekeeping_cpumask(HK_FLAG_DOMAIN)

few lines higher. I.e. the fixup adds redundant check against the empty
sched domain production.

Sorry for the noise and HTH,
Michal

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