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Date:   Thu, 17 Dec 2020 14:44:01 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Will Deacon <will@kernel.org>,
        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>,
        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>,
        kernel-team@android.com
Subject: Re: [PATCH v5 07/15] cpuset: Don't use the cpu_possible_mask as a
 last resort for cgroup v1
Message-ID: <20201217134401.GY3040@hirez.programming.kicks-ass.net>
References: <20201208132835.6151-1-will@kernel.org>
 <20201208132835.6151-8-will@kernel.org>
 <20201217121552.ds7g2icvqp5nvtha@e107158-lin.cambridge.arm.com>
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On Thu, Dec 17, 2020 at 12:15:52PM +0000, Qais Yousef wrote:
> On 12/08/20 13:28, Will Deacon wrote:
> > If the scheduler cannot find an allowed CPU for a task,
> > cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
> > if cgroup v1 is in use.
> > 
> > In preparation for allowing architectures to provide their own fallback
> > mask, just return early if we're not using cgroup v2 and allow
> > select_fallback_rq() to figure out the mask by itself.
> > 
> > Cc: Li Zefan <lizefan@huawei.com>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Reviewed-by: Quentin Perret <qperret@google.com>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  kernel/cgroup/cpuset.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 57b5b5d0a5fd..e970737c3ed2 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -3299,9 +3299,11 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
> >  
> >  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
> >  {
> > +	if (!is_in_v2_mode())
> > +		return; /* select_fallback_rq will try harder */
> > +
> >  	rcu_read_lock();
> > -	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> > -		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> > +	do_set_cpus_allowed(tsk, task_cs(tsk)->cpus_allowed);
> 
> Why is it safe to return that for cpuset v2?

v1

Because in that case it does cpu_possible_mask, which, if you look at
select_fallback_rq(), is exactly what happens when cpuset 'fails' to
find a candidate.

Or at least, that's how I read the patch.