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Date:   Mon, 24 May 2021 21:21:32 +0100
From:   Will Deacon <will@kernel.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, kernel-team@android.com
Subject: Re: [PATCH v6 07/21] cpuset: Don't use the cpu_possible_mask as a
 last resort for cgroup v1
Message-ID: <20210524202131.GB15545@willie-the-truck>
References: <20210518094725.7701-1-will@kernel.org>
 <20210518094725.7701-8-will@kernel.org>
 <20210521173934.pjcv37j63odtsrp6@e107158-lin.cambridge.arm.com>
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Hi Qais,

On Fri, May 21, 2021 at 06:39:34PM +0100, Qais Yousef wrote:
> On 05/18/21 10:47, Will Deacon wrote:
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index a945504c0ae7..8c799260a4a2 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -3322,9 +3322,17 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
> >  
> >  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
> >  {
> > +	const struct cpumask *cs_mask;
> > +	const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
> > +
> >  	rcu_read_lock();
> > -	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> > -		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> > +	cs_mask = task_cs(tsk)->cpus_allowed;
> > +
> > +	if (!is_in_v2_mode() || !cpumask_subset(cs_mask, possible_mask))
> > +		goto unlock; /* select_fallback_rq will try harder */
> > +
> > +	do_set_cpus_allowed(tsk, cs_mask);
> 
> Shouldn't we take the intersection with possible_mask like we discussed before?
> 
> 	https://lore.kernel.org/lkml/20201217145954.GA17881@willie-the-truck/

Yes, and that's what the '!cpumask_subset()' check is doing above. Either
we use the valid subset of the cpuset mask (which is the intersection with
the possible mask) or we bail if that set is empty.

Will