linux-kernel - Re: [PATCH] mm: vmpressure: don't count userspace-induced reclaim as memory pressure

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Date:   Thu, 23 Jun 2022 19:04:23 +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>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: vmpressure: don't count userspace-induced reclaim as
 memory pressure

On Thu 23-06-22 09:42:43, Shakeel Butt wrote:
> On Thu, Jun 23, 2022 at 9:37 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Thu 23-06-22 09:22:35, Yosry Ahmed wrote:
> > > On Thu, Jun 23, 2022 at 2:43 AM Michal Hocko <mhocko@...e.com> wrote:
> > > >
> > > > On Thu 23-06-22 01:35:59, Yosry Ahmed wrote:
> > [...]
> > > > > In our internal version of memory.reclaim that we recently upstreamed,
> > > > > we do not account vmpressure during proactive reclaim (similar to how
> > > > > psi is handled upstream). We want to make sure this behavior also
> > > > > exists in the upstream version so that consolidating them does not
> > > > > break our users who rely on vmpressure and will start seeing increased
> > > > > pressure due to proactive reclaim.
> > > >
> > > > These are good reasons to have this patch in your tree. But why is this
> > > > patch benefitial for the upstream kernel? It clearly adds some code and
> > > > some special casing which will add a maintenance overhead.
> > >
> > > It is not just Google, any existing vmpressure users will start seeing
> > > false pressure notifications with memory.reclaim. The main goal of the
> > > patch is to make sure memory.reclaim does not break pre-existing users
> > > of vmpressure, and doing it in a way that is consistent with psi makes
> > > sense.
> >
> > memory.reclaim is v2 only feature which doesn't have vmpressure
> > interface. So I do not see how pre-existing users of the upstream kernel
> > can see any breakage.
> >
> 
> Please note that vmpressure is still being used in v2 by the
> networking layer (see mem_cgroup_under_socket_pressure()) for
> detecting memory pressure.

I have missed this. It is hidden quite good. I thought that v2 is
completely vmpressure free. I have to admit that the effect of
mem_cgroup_under_socket_pressure is not really clear to me. Not to
mention whether it should or shouldn't be triggered for the user
triggered memory reclaim. So this would really need some explanation.
 
> Though IMO we should deprecate vmpressure altogether.

Yes it should be really limited to v1. But as I've said the effect on
mem_cgroup_under_socket_pressure is not really clear to me. It really
seems the v2 support has been introduced deliberately.

-- 
Michal Hocko
SUSE Labs