linux-kernel - Re: [PATCH v4] mm: vmpressure: don't count proactive reclaim in vmpressure

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Message-ID: <Ytk7octCv1DZeelM@dhcp22.suse.cz>
Date:   Thu, 21 Jul 2022 13:42:25 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>, NeilBrown <neilb@...e.de>,
        Alistair Popple <apopple@...dia.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH v4] mm: vmpressure: don't count proactive reclaim in
 vmpressure

On Wed 20-07-22 10:49:53, Shakeel Butt wrote:
> On Wed, Jul 20, 2022 at 2:24 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> [...]
> >
> > I think what we are missing here is
> > - explain that this doesn't have any effect on existing users of
> >   vmpressure user interface because that is cgroup v1 and memory.reclaim
> >   is v2 feature. This is a trivial statement but quite useful for future
> >   readers of this commit
> > - explain the effect on the networking layer and typical usecases
> >   memory.reclaim is used for currently and ideally document that.
> 
> I agree with the above two points (Yosry, please address those) but
> the following third point is orthogonal and we don't really need to
> have an answer for this patch to be accepted.
> 
> > - how are we going to deal with users who would really want to use
> >   memory.reclaim interface as a replacement for existing hard/high
> >   memory reclaim? Is that even something that the interface is intended
> >   for?
> 
> I do agree that this question is important. Nowadays I am looking at
> this from a different perspective and use-case. More concretely how
> (and why) to replace vmpressure based network throttling for cgroup
> v2. I will start a separate thread for that discussion.

I think we should be good to document this side effect for now. If you
have a plan to change to vmpressure based throttling then only better.
But one way or the other impact of the memory.reclaim interface on
netwroking should be documented properly.

-- 
Michal Hocko
SUSE Labs