Re: [PATCH] sched/mmcid: Defer affinity updates until schedule-in

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] sched/mmcid: affinity expansions lost or delayed by
  mm_cid_schedin() deferral
--

commit 8130aa795d763d1b0e9fc8f80bab96df5fedde9e
Author: Weiming Shi <bestswngs@gmail.com>

sched/mmcid: Defer affinity updates until schedule-in

This commit defers MM-CID allowed-CPU mask updates from
set_cpus_allowed_common() to mm_cid_schedin() and sched_mm_cid_fork() to
avoid accessing a remote task's mm while it may be replaced and freed
during exec.

> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 7d2ec527b8a25..8e8934376e340 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
[ ... ]
> @@ -4132,6 +4139,10 @@ static __always_inline void mm_cid_schedin(struct task_struct *next)
>  
>  	if (!next->mm_cid.active)
>  		return;
> +	if (unlikely(next->mm_cid.cpus_allowed_changed)) {
> +		mm_update_cpus_allowed(mm, &next->cpus_mask);
> +		next->mm_cid.cpus_allowed_changed = false;
> +	}
>  
>  	cpu_cid = __this_cpu_read(mm->mm_cid.pcpu->cid);
>  	mode = READ_ONCE(mm->mm_cid.mode);

[Severity: Medium]
Can deferring mm_update_cpus_allowed() solely to mm_cid_schedin() delay or
lose affinity expansions in mm_cpus_allowed(mm)?

When a running task widens its affinity mask, task_cpu(p) remains in
p->cpus_mask, so affine_move_task() returns without migrating or
context-switching p.  Until p is later preempted and scheduled back in,
mm->mm_cid.nr_cpus_allowed, max_cids, and pcpu_thrs stay stale for all
threads sharing the mm.

Because p->cpus_mask only holds the latest mask, any intermediate
expansion that is narrowed again before the next mm_cid_schedin() (or if
the task exits via sched_mm_cid_exit() before scheduling in again) is
never folded into the growth-only mm_cpus_allowed(mm) mask, leaving
max_cids capped too low or triggering wrong per-CPU mode transitions.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261106120846.2405936-1-bestswngs@gmail.com?part=1