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Date:   Wed, 31 Aug 2022 11:21:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v6 3/5] sched: Enforce user requested affinity
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References: <20220826010119.1265764-1-longman@redhat.com>
 <20220826010119.1265764-4-longman@redhat.com>
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On Wed, Aug 31, 2022 at 11:18:22AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 25, 2022 at 09:01:17PM -0400, Waiman Long wrote:
> 
> > @@ -9352,6 +9381,11 @@ int sched_cpu_activate(unsigned int cpu)
> >  		sched_update_numa(cpu, true);
> >  		sched_domains_numa_masks_set(cpu);
> >  		cpuset_cpu_active();
> > +		/*
> > +		 * Preallocated scratch cpumask
> > +		 */
> > +		if (!rq->scratch_mask)
> > +			rq->scratch_mask = kmalloc(cpumask_size(), GFP_KERNEL);
> >  	}
> 
> this is too late; I think you'll have to add a sched_cpu_prepare() and
> simply fail the cpu-up when the allocation fails.

Alternatively, waste some more memory and add yet another per-cpu
cpumask.