linux-kernel - Re: [PATCH] ipc: remove memcg accounting for sops objects in do

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Message-ID: <20210914142316.GA23024@blackbody.suse.cz>
Date:   Tue, 14 Sep 2021 16:23:16 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Vasily Averin <vvs@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>, kernel@...nvz.org,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH] ipc: remove memcg accounting for sops objects in
 do_semtimedop()

On Tue, Sep 14, 2021 at 09:13:48AM +0200, Michal Hocko <mhocko@...e.com> wrote:
> "
> This object can consume up to 2 pages, syscall is sleeping one,
> size and duration can be controlled by user, and this allocation
> can be repeated by many thread at the same time.
> "
> 
> It sounds like a problem, except it is not because? A worst case
> scenario evaluation would be beneficial for example

AFAICS, the offending allocation is in place only during the duration of
the syscall. So it's basically O(#tasks).
Considering at least 2 pages for task_struct + 2 pages for kernel stack,
back of the envelope calculation gives me the footprint amplification is
<1.5.
The factor would IMO be interesting if it was >> 2 (from the PoV of
excessive (ab)use, fine-grained accounting seems to be currently
unfeasible due to performance impact).

The commit message can be more explicit about this but to the patch
Reviewed-by: Michal Koutný <mkoutny@...e.com>