Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 14 Sep 2021 16:23:16 +0200
From:   Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Vasily Averin <vvs@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>, kernel@openvz.org,
        Cgroups <cgroups@vger.kernel.org>
Subject: Re: [PATCH] ipc: remove memcg accounting for sops objects in
 do_semtimedop()
Message-ID: <20210914142316.GA23024@blackbody.suse.cz>
References: <90e254df-0dfe-f080-011e-b7c53ee7fd20@virtuozzo.com>
 <YT8NrsaztWNDpKXk@dhcp22.suse.cz>
 <CALvZod7Y4pC4XvqVp+tJ==CnS5Ay8YPqrxeUzA8tMLu+0U3hjQ@mail.gmail.com>
 <YUBLrJOL6DGxmira@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <YUBLrJOL6DGxmira@dhcp22.suse.cz>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

On Tue, Sep 14, 2021 at 09:13:48AM +0200, Michal Hocko <mhocko@suse.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 Koutn? <mkoutny@suse.com>