linux-kernel - Re: [PATCH 0/3] memcg: non-unified flushing for userspace stats

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Message-ID: <a4opjlmm5it3ujoypcgjfljfamjvr7gu34sc7lrjldfbqzz4lj@6w4lqdcfd3zu>
Date:   Tue, 22 Aug 2023 15:00:55 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        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 0/3] memcg: non-unified flushing for userspace stats

Hello.

On Mon, Aug 21, 2023 at 08:54:55PM +0000, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> For userspace reads, unified flushing leads to non-deterministic stats
> staleness and reading cost.

I only skimed previous threads but I don't remember if it was resolved:
a) periodic flushing was too much spaced for user space readers (i.e. 2s
   delay is too much [1]),
b) periodic flushing didn't catch up (i.e. full tree flush can
   occassionaly take more than 2s) leading to extra staleness?

[1] Assuming that nr_cpus*MEMCG_CHARGE_BATCH error bound is also too
    much for userspace readers, correct?

> The cost of userspace reads are now determinstic, and depend on the
> size of the subtree being read. This should fix both the *sometimes*
> expensive reads (due to flushing the entire tree) and occasional
> staless (due to skipping flushing).

This is nice, thanks to the atomic removal in the commit 0a2dc6ac3329
("cgroup: remove cgroup_rstat_flush_atomic()"). I think the smaller
chunks with yielding could be universally better (last words :-).

Thanks,
Michal


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