Re: [PATCH] cgroup/cpuset: Don't filter offline CPUs in cpuset_cpus_allowed() for top cpuset tasks

[Tejun Heo]

On Fri, Feb 03, 2023 at 11:40:40AM -0500, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user
> requested cpumask"), relax_compatible_cpus_allowed_ptr() is calling
> __sched_setaffinity() unconditionally. This helps to expose a bug in
> the current cpuset hotplug code where the cpumasks of the tasks in
> the top cpuset are not updated at all when some CPUs become online or
> offline. It is likely caused by the fact that some of the tasks in the
> top cpuset, like percpu kthreads, cannot have their cpu affinity changed.
> 
> One way to reproduce this as suggested by Peter is:
>  - boot machine
>  - offline all CPUs except one
>  - taskset -p ffffffff $$
>  - online all CPUs
> 
> Fix this by allowing cpuset_cpus_allowed() to return a wider mask that
> includes offline CPUs for those tasks that are in the top cpuset. For
> tasks not in the top cpuset, the old rule applies and only online CPUs
> will be returned in the mask since hotplug events will update their
> cpumasks accordingly.
> 
> Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")
> Reported-by: Will Deacon <will@xxxxxxxxxx>
> Originally-from: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

So, this is the replacement for the first patch[1] Will posted, right?

>  void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  {
>  	unsigned long flags;
> +	struct cpuset *cs;
>  
>  	spin_lock_irqsave(&callback_lock, flags);
> -	guarantee_online_cpus(tsk, pmask);
> +	rcu_read_lock();
> +
> +	cs = task_cs(tsk);
> +	if (cs != &top_cpuset)
> +		guarantee_online_cpus(tsk, pmask);
> +	/*
> +	 * TODO: Tasks in the top cpuset won't get update to their cpumasks
> +	 * when a hotplug online/offline event happens. So we include all
> +	 * offline cpus in the allowed cpu list.
> +	 */
> +	if ((cs == &top_cpuset) || cpumask_empty(pmask)) {
> +		const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
> +
> +		/*
> +		 * We first exclude cpus allocated to partitions. If there is no
> +		 * allowable online cpu left, we fall back to all possible cpus.
> +		 */
> +		cpumask_andnot(pmask, possible_mask, top_cpuset.subparts_cpus);

and the differences are that

* It's only applied to the root cgroup.

* Cpus taken up by partitions are excluded.

Is my understanding correct?

> +		if (!cpumask_intersects(pmask, cpu_online_mask))
> +			cpumask_copy(pmask, possible_mask);
> +	}
> +
> +	rcu_read_unlock();
>  	spin_unlock_irqrestore(&callback_lock, flags);

So, I suppose you're suggesting applying this patch instead of the one Will
Deacon posted[1] and we need Will's second patch[2] on top, right?

[1] http://lkml.kernel.org/r/20230131221719.3176-3-will@xxxxxxxxxx
[2] http://lkml.kernel.org/r/20230131221719.3176-3-will@xxxxxxxxxx

Thanks.

-- 
tejun