linux-kernel - Re: [RFC PATCH] sched: Do not copy user_cpus_ptr when parent is reset_on

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Message-ID: <a645086d-bffb-41b0-bd70-1ef5edb128f9@redhat.com>
Date: Thu, 5 Sep 2024 08:42:33 -0400
From: Waiman Long <longman@...hat.com>
To: Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org
Cc: dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
 ke.wang@...soc.com, di.shen@...soc.com, xuewen.yan94@...il.com
Subject: Re: [RFC PATCH] sched: Do not copy user_cpus_ptr when parent is
 reset_on_fork

On 9/5/24 05:04, Xuewen Yan wrote:
> Now, the task's user_cpus_ptr would dup from parent's user_cpus_ptr.
> It is better reset the user_cpus_ptr when parent's reset_on_fork
> is set.

According to sched(7):

        Each thread has a reset-on-fork scheduling flag.  When this flag
        is set, children created by fork(2) do not inherit privileged
        scheduling policies.

It can be argued what are considered privileged scheduling policies. 
AFAICS, a restricted affinity doesn't seem to be a "privileged" 
scheduling policy. That is my own opinion strictly from the definition 
point of view, I will let others weigh in on that and I am OK to go 
either way.

Cheers,
Longman

>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
>   kernel/sched/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f3951e4a55e5..2fbae00cd1dc 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2666,7 +2666,7 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>   	 * do_set_cpus_allowed().
>   	 */
>   	raw_spin_lock_irqsave(&src->pi_lock, flags);
> -	if (src->user_cpus_ptr) {
> +	if (src->user_cpus_ptr && !src->sched_reset_on_fork) {
>   		swap(dst->user_cpus_ptr, user_mask);
>   		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
>   	}