Re: [RFC PATCH] sched: Do not copy user_cpus_ptr when parent is reset_on_fork

[Phil Auld]

On Thu, Sep 05, 2024 at 08:42:33AM -0400 Waiman Long wrote:
> 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.
>

I think that one could argue that clearing a restricted affinity is
increasing the privilege and not preventing inheritence of same.
i.e. it would be the opposite of what reset-on-fork means.

I'd say NAK to this one if I had that power.

Cheers,
Phil

> Cheers,
> Longman
> 
> > 
> > Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> > ---
> >   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);
> >   	}
> 
> 

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