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

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Date:   Thu, 19 May 2022 05:08:35 +0000
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Vaibhav Jain <vaibhav@...ux.ibm.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'

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.

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.