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

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Date:   Thu, 19 May 2022 15:26:52 +0530
From:   Vaibhav Jain <vaibhav@...ux.ibm.com>
To:     Greg Thelen <gthelen@...gle.com>, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     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>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yosry Ahmed <yosryahmed@...gle.com>
Subject: Re: [PATCH] memcg: provide reclaim stats via 'memory.reclaim'

Hi,

Thanks for looking into this patch,

Greg Thelen <gthelen@...gle.com> writes:

> Vaibhav Jain <vaibhav@...ux.ibm.com> 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
>
> I certainly appreciate the ability for shell scripts to demonstrate
> cgroup operations with textual interfaces, but such interface seem like
> they are optimized for ease of use by developers.
>
Agree that directly exposing nr_scanned/reclaimed might not be a useful
for users and certainly looks like a dev interface

> I wonder if for runtime production use an ioctl or netlink interface has
> been considered for cgroup? I don't think there are any yet, but such
> approaches seem like a more straightforward ways to get nontrivial
> input/outputs from a single call (e.g. like this proposal). And they
> have the benefit of not requiring ascii serialization/parsing overhead.

I think to a large degree eBPF and existing static tracepoints in vmscan
can provide access to these metrics as Shakeel Bhat pointed to earlier.

<snip>

-- 
Cheers
~ Vaibhav