Re: [RFC PATCH v3] sched: Fix performance regression introduced by mm_cid

[Mathieu Desnoyers]

On 2023-04-11 05:37, Peter Zijlstra wrote:
> 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.

AFAIU the finish_task_switch()'s mmdrop() is for the case where:

                 * [...] or in
                 * case 'prev->active_mm == next->mm' through
                 * finish_task_switch()'s mmdrop().

which applies for the case where we schedule from a kernel thread (which
kept the prior user task's mm as active mm) to a user task with the same
mm.

But this is really a transition from kernel -> user, not user -> user ?

Why should either membarrier or mm_cid care about a transition from
prev->mm to next->mm where mm is unchanged ? It does not register
as a transition from the comparison perspective.

I'll update my comment in switch_mm_cid to:

     /*
      * user -> user transition guarantees a memory barrier through
      * switch_mm() when current->mm changes. If current->mm is
      * unchanged, no barrier is needed.
      */

Thanks,

Mathieu


> > +	}
> >   	if (prev->mm_cid_active) {
> >   		mm_cid_put_lazy(prev);
> >   		prev->mm_cid = -1;

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com