linux-kernel - Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads

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Date:   Mon, 28 Aug 2023 11:35:41 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for
 userspace reads

On Mon, Aug 28, 2023 at 10:43 AM Waiman Long <longman@...hat.com> wrote:
>
>
> On 8/28/23 13:35, Waiman Long wrote:
> > On 8/28/23 13:28, Yosry Ahmed wrote:
> >> On Mon, Aug 28, 2023 at 10:27 AM Waiman Long <longman@...hat.com> wrote:
> >>>
> >>> On 8/28/23 13:07, Yosry Ahmed wrote:
> >>>>> Here I agree with you. Let's go with the approach which is easy to
> >>>>> undo for now. Though I prefer the new explicit interface for
> >>>>> flushing,
> >>>>> that step would be very hard to undo. Let's reevaluate if the
> >>>>> proposed
> >>>>> approach shows negative impact on production traffic and I think
> >>>>> Cloudflare folks can give us the results soon.
> >>>> Do you prefer we also switch to using a mutex (with preemption
> >>>> disabled) to avoid the scenario Michal described where flushers give
> >>>> up the lock and sleep resulting in an unbounded wait time in the worst
> >>>> case?
> >>> Locking with mutex with preemption disabled is an oxymoron. Use
> >>> spinlock
> >>> if you want to have preemption disabled. The purpose of usiing mutex is
> >>> to allow the lock owner to sleep, but you can't sleep with preemption
> >>> disabled. You need to enable preemption first. You can disable
> >>> preemption for a short time in a non-sleeping section of the lock
> >>> critical section, but I would not recommend disabling preemption for
> >>> the
> >>> whole critical section.
> >> I thought using a mutex with preemption disabled would at least allow
> >> waiters to sleep rather than spin, is this not correct (or doesn't
> >> matter) ?
> >
> > Because of optimistic spinning, a mutex lock waiter will only sleep if
> > the lock holder sleep or when its time slice run out. So the waiters
> > are likely to spin for quite a while before they go to sleep.

I see. Thanks for the explanation.

>
> Perhaps you can add a mutex at the read side so that only 1 reader can
> contend with the global rstat spinlock at any time if this is a concern.

I guess we can keep it simple for now and add that later if needed.
For unified flushers we already can only have one. If we see a problem
from the stat reading side we can add a mutex there.

Thanks!