From mboxrd@z Thu Jan  1 00:00:00 1970
From: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for
 userspace reads
Date: Mon, 28 Aug 2023 13:35:08 -0400
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To: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>, Muchun Song <muchun.song-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>, Ivan Babrou <ivan-lDpJ742SOEtZroRs9YW3xA@public.gmane.org>, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

On 8/28/23 13:28, Yosry Ahmed wrote:
> On Mon, Aug 28, 2023 at 10:27=E2=80=AFAM Waiman Long <longman-H+wXaHxf7aL=
QT0dZR+AlfA@public.gmane.org> 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=20
the lock holder sleep or when its time slice run out. So the waiters are=20
likely to spin for quite a while before they go to sleep.

Cheers,
Longman