linux-kernel - Re: [PATCH v2 1/2] mm: memcg: refactor page state unit helpers

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Message-ID: <4h5uae72ti6jyiibcyfg2bytooy6d6ggtkrgod5a6rmpateyra@4setu5jmd5kn>
Date:   Thu, 5 Oct 2023 18:30:25 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: memcg: refactor page state unit helpers

On Thu, Oct 05, 2023 at 02:31:03AM -0700, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> I am not really sure what you mean here.

My "vision" is to treat WORKINGSET_ entries as events.
That would mean implementing per-node tracking for vm_event_item
(costlier?).
That would mean node_stat_item and vm_event_item being effectively
equal, so they could be merged in one.
That would be situation to come up with new classification based on use
cases (e.g. precision/timeliness requirements, state vs change
semantics).

(Do not take this as blocker of the patch 1/2, I rather used the
opportunity to discuss a greater possible cleanup.)

> We don't track things like OOM_KILL and DROP_PAGECACHE per memcg as
> far as I can tell.

Ah, good. (I forgot only subset of entries is relevant for memcgs.)

> This will mean that WORKINGSET_* state will become more stale. We will
> need 4096 as many updates as today to get a flush. These are used by
> internal flushers (reclaim), and are exposed to userspace. I am not
> sure we want to do that.

snapshot_refaults() doesn't seem to follow after flush
and
workigset_refault()'s flush doesn't seem to preceed readers

Is the flush misplaced or have I overlooked something?
(If the former, it seems to work good enough even with the current
flushing heuristics :-))


Michal

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