linux-kernel - Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

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Message-ID: <CAJD7tkYgCySTX28zK9GZiWwsabR4nv7M2hQ57y12si-fqtv7zg@mail.gmail.com>
Date:   Mon, 14 Aug 2023 17:50:46 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>, Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <muchun.song@...ux.dev>, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

On Mon, Aug 14, 2023 at 5:48 PM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Mon, Aug 14, 2023 at 05:39:15PM -0700, Yosry Ahmed wrote:
> > I believe dropping unified flushing, if possible of course, may fix
> > both problems.
>
> Yeah, flushing the whole tree for every stat read will push up the big O
> complexity of the operation. It shouldn't be too bad because only what's
> updated since the last read will need flushing but if you have a really big
> machine with a lot of constantly active cgroups, you're still gonna feel it.
> So, yeah, drop that and switch the global lock to mutex and we should all be
> good?

I hope so, but I am not sure.

The unified flushing was added initially to mitigate a thundering herd
problem from concurrent in-kernel flushers (e.g. concurrent reclaims),
but back then flushing was atomic so we had to keep the spinlock held
for a long time. I think it should be better now, but I am hoping
Shakeel will chime in since he added the unified flushing originally.

We also need to agree on what to do about stats_flushing_threshold and
flush_next_time since they're both global now (since all flushing is
global).

>
> Thanks.
>
> --
> tejun