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

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Date:   Tue, 29 Aug 2023 17:17:24 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Shakeel Butt <shakeelb@...gle.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 Tue 29-08-23 11:05:28, Waiman Long wrote:
> On 8/29/23 03:27, Michal Hocko wrote:
> > On Mon 28-08-23 13:27:23, Waiman Long 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.
> > I believe Yosry wanted to disable preemption _after_ the lock is taken
> > to reduce the time spent while it is held. The idea to use the mutex is
> > to reduce spinning and more importantly to get rid of lock dropping
> > part. It is not really clear (but unlikely) we can drop it while
> > preserving the spinlock as the thing scales with O(#cgroups x #cpus)
> > in the worst case.
> 
> As I have said later in my email, I am not against disabling preemption
> selectively on some parts of the lock critical section where preemption is
> undesirable. However, I am against disabling preemption for the whole
> duration of the code where the mutex lock is held as it defeats the purpose
> of using mutex in the first place.

I certainly agree this is an antipattern.
-- 
Michal Hocko
SUSE Labs