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

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Message-ID: <820843a5-d7a5-91a9-b861-99e7132ddb98@redhat.com>
Date:   Mon, 28 Aug 2023 13:43:08 -0400
From:   Waiman Long <longman@...hat.com>
To:     Yosry Ahmed <yosryahmed@...gle.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 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.

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.

Regards,
Longman