linux-kernel - Re: [PATCH 2/5] cgroup/cpuset: Include offline CPUs when tasks' cpumasks in top

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Date:   Tue, 14 Mar 2023 18:34:11 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/5] cgroup/cpuset: Include offline CPUs when tasks'
 cpumasks in top_cpuset are updated

Hello Waiman.

On Mon, Mar 06, 2023 at 03:08:46PM -0500, Waiman Long <longman@...hat.com> wrote:
> -		/*
> -		 * Percpu kthreads in top_cpuset are ignored
> -		 */
> -		if (top_cs && (task->flags & PF_KTHREAD) &&
> -		    kthread_is_per_cpu(task))
> -			continue;
> +		const struct cpumask *possible_mask = task_cpu_possible_mask(task);
>  
> -		cpumask_and(new_cpus, cs->effective_cpus,
> -			    task_cpu_possible_mask(task));
> +		if (top_cs) {
> +			/*
> +			 * Percpu kthreads in top_cpuset are ignored
> +			 */
> +			if ((task->flags & PF_KTHREAD) && kthread_is_per_cpu(task))
> +				continue;
> +			cpumask_andnot(new_cpus, possible_mask, cs->subparts_cpus);
> +		} else {
> +			cpumask_and(new_cpus, cs->effective_cpus, possible_mask);
> +		}

I'm wrapping my head around this slightly.
1) I'd suggest swapping args in of cpumask_and() to have possible_mask
   consistently first.
2) Then I'm wondering whether two branches are truly different when
   effective_cpus := cpus_allowed - subparts_cpus
   top_cpuset.cpus_allowed == possible_mask        (1)

IOW, can you see a difference in what affinities are set to eligible
top_cpuset tasks before and after this patch upon CPU hotplug?
(Hm, (1) holds only in v2. So is this a fix for v1 only?)

Thanks,
Michal

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