linux-kernel - Re: [PATCH v6 3/5] sched: Enforce user requested affinity

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Message-ID: <4ac0cfd5-cf1e-9bcd-7476-494bfbeb1c50@redhat.com>
Date:   Wed, 31 Aug 2022 17:00:16 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v6 3/5] sched: Enforce user requested affinity


On 8/31/22 05:21, Peter Zijlstra wrote:
> 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.

A percpu cpumask is probably a better idea. However, I don't need that 
as early as the other cpumasks like __cpu_possible_mask. Maybe I can do 
a percpu memory allocation early in the pre-smp boot.

Cheers,
Longman