linux-kernel - Re: [PATCH] memcg: provide reclaim stats via 'memory.reclaim'

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Date:   Thu, 19 May 2022 15:11:41 +0530
From:   Vaibhav Jain <vaibhav@...ux.ibm.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Yosry Ahmed <yosryahmed@...gle.com>
Subject: Re: [PATCH] memcg: provide reclaim stats via 'memory.reclaim'

Hi,

Thanks for looking into this patch,

Shakeel Butt <shakeelb@...gle.com> writes:

> On Thu, May 19, 2022 at 04:08:15AM +0530, Vaibhav Jain wrote:
>> [1] Provides a way for user-space to trigger proactive reclaim by introducing
>> a write-only memcg file 'memory.reclaim'. However reclaim stats like number
>> of pages scanned and reclaimed is still not directly available to the
>> user-space.
>> 
>> This patch proposes to extend [1] to make the memcg file 'memory.reclaim'
>> readable which returns the number of pages scanned / reclaimed during the
>> reclaim process from 'struct vmpressure' associated with each memcg. This should
>> let user-space asses how successful proactive reclaim triggered from memcg
>> 'memory.reclaim' was ?
>> 
>> With the patch following command flow is expected:
>> 
>>  # echo "1M" > memory.reclaim
>> 
>>  # cat memory.reclaim
>>    scanned 76
>>    reclaimed 32
>> 
>
> Yosry already mentioned the race issue with the implementation and I
> would prefer we don't create any new dependency on vmpressure which I
> think we should deprecate.
Ok,

>
> Anyways my question is how are you planning to use these metrics i.e.
> scanned & reclaimed? I wonder if the data you are interested in can be
> extracted without a stable interface. Have you tried BPF way to get
> these metrics? We already have a tracepoint in vmscan tracing the
> scanned and reclaimed. 
>
Agree that there are enough static trace_mm_vmscan_ tracepoints in
vmscan to get that info.

Also agree that exposing nr_scanned/nr_reclaimed directly to userspace may not
be a good idea but knowing the amount of memory reclaimed might be
useful.

With user-space triggered proactive reclaim user-space code can try to
write a certain value to "memory.reclaim" in a loop till it returns
'-EBUSY'.

Right now there is no direct way for it to get feedback on the progress
of the requested reclaim. Providing a stable interface to ascertain the
progress of reclaim lets that userspace provide smaller values for
proactive reclaim 

-- 
Cheers
~ Vaibhav