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Message-ID: <20230411093705.GB578657@hirez.programming.kicks-ass.net>
Date: Tue, 11 Apr 2023 11:37:05 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>,
Olivier Dion <odion@...icios.com>, michael.christie@...cle.com
Subject: Re: [RFC PATCH v3] sched: Fix performance regression introduced by
mm_cid
On Fri, Apr 07, 2023 at 09:14:36PM -0400, Mathieu Desnoyers wrote:
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index bc0e1cd0d6ac..f3e7dc2cd1cc 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3354,6 +3354,37 @@ static inline int mm_cid_get(struct mm_struct *mm)
> static inline void switch_mm_cid(struct task_struct *prev, struct task_struct *next)
> {
> + /*
> + * Provide a memory barrier between rq->curr store and load of
> + * {prev,next}->mm->pcpu_cid[cpu] on rq->curr->mm transition.
> + *
> + * Should be adapted if context_switch() is modified.
> + */
> + if (!next->mm) { // to kernel
> + /*
> + * user -> kernel transition does not guarantee a barrier, but
> + * we can use the fact that it performs an atomic operation in
> + * mmgrab().
> + */
> + if (prev->mm) // from user
> + smp_mb__after_mmgrab();
> + /*
> + * kernel -> kernel transition does not change rq->curr->mm
> + * state. It stays NULL.
> + */
> + } else { // to user
> + /*
> + * kernel -> user transition does not provide a barrier
> + * between rq->curr store and load of {prev,next}->mm->pcpu_cid[cpu].
> + * Provide it here.
> + */
> + if (!prev->mm) // from kernel
> + smp_mb();
> + /*
> + * user -> user transition guarantees a memory barrier through
> + * switch_mm().
> + */
What about the user->user case where next->mm == prev->mm ? There
sys_membarrier() relies on finish_task_switch()'s mmdrop(), but we
can't.
> + }
> if (prev->mm_cid_active) {
> mm_cid_put_lazy(prev);
> prev->mm_cid = -1;
>
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