linux-kernel - Re: [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask in sched

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Message-ID: <21ade241-76b9-4f0a-8e99-be033dcc882c@redhat.com>
Date: Mon, 20 Oct 2025 22:13:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, 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>, Valentin Schneider <vschneid@...hat.com>,
 Jonathan Corbet <corbet@....net>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-doc@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Catalin Marinas <catalin.marinas@....com>, Nico Pache <npache@...hat.com>,
 Phil Auld <pauld@...hat.com>, John Coleman <jocolema@...hat.com>
Subject: Re: [PATCH 1/2] sched/core: Enable full cpumask to clear user cpumask
 in sched_setaffinity()

On 23.09.25 19:54, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> perserved even if the task is being moved to a different cpuset.
> However, that affinity is also being inherited by any subsequently
> created child processes which may not want or be aware of that affinity.

So I assume setting the affinity to the full bitmap would then allow any 
child to essentially reset to the default, correct?

> 
> One way to solve this problem is to provide a way to back off from
> that user provided CPU affinity.  This patch implements such a scheme
> by using a full cpumask (a cpumask with all bits set) to signal the
> clearing of the user cpumask to follow the default as allowed by
> the current cpuset.  In fact, with a full cpumask in user_cpus_ptr,
> the task behavior should be the same as with a NULL user_cpus_ptr.
> This patch just formalizes it without causing any incompatibility and
> discard an otherwise useless cpumask.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>   kernel/sched/syscalls.c | 20 ++++++++++++++------
>   1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 77ae87f36e84..d68c7a4ee525 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1229,14 +1229,22 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>   		return retval;
>   
>   	/*
> -	 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> -	 * alloc_user_cpus_ptr() returns NULL.
> +	 * If a full cpumask is passed in, clear user_cpus_ptr and reset the
> +	 * current cpu affinity to the default for the current cpuset.
>   	 */
> -	user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> -	if (user_mask) {
> -		cpumask_copy(user_mask, in_mask);
> +	if (cpumask_full(in_mask)) {
> +		user_mask = NULL;
>   	} else {
> -		return -ENOMEM;
> +		/*
> +		 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> +		 * alloc_user_cpus_ptr() returns NULL.
> +		 */
> +		user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> +		if (user_mask) {
> +			cpumask_copy(user_mask, in_mask);
> +		} else {
> +			return -ENOMEM;
> +		}
>   	}
>   
>   	ac = (struct affinity_context){

Not an expert on this code.

I'm only wondering if there is somehow, some way we could be breaking 
user space by doing that.


-- 
Cheers

David / dhildenb