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Date:   Thu, 17 Dec 2020 15:00:48 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
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: <20201217150048.d6enq5hhchvh32hz@e107158-lin.cambridge.arm.com>
References: <20201208132835.6151-1-will@kernel.org>
 <20201208132835.6151-8-will@kernel.org>
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On 12/17/20 14:44, Peter Zijlstra wrote:
> 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.

Okay I can see that if v2 has effectively empty mask for the 32bit tasks, then
we'll fallback to the 'possible' switch case where we set
task_cpu_possible_mask().

But how about when task_cs(tsk)->cpus_allowed contains partially invalid cpus?

The search for a candidate cpu will return a correct dest_cpu, but the actual
cpu_mask of the task will contain invalid cpus that could be picked up later,
no? Shouldn't we

	cpumask_and(mask, task_cs(tsk)->cpus_allowed, task_cpu_possible_mask())

to remove those invalid cpus?

Thanks

--
Qais Yousef