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Date:   Mon, 27 Jun 2022 10:25:21 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Miaohe Lin <linmiaohe@huawei.com>, NeilBrown <neilb@suse.de>,
        Alistair Popple <apopple@nvidia.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Peter Xu <peterx@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: vmpressure: don't count userspace-induced reclaim as
 memory pressure
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On Thu 23-06-22 10:26:11, Yosry Ahmed wrote:
> On Thu, Jun 23, 2022 at 10:04 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 23-06-22 09:42:43, Shakeel Butt wrote:
> > > On Thu, Jun 23, 2022 at 9:37 AM Michal Hocko <mhocko@suse.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@suse.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.
> 
> vmpressure was tied into socket pressure by 8e8ae645249b ("mm:
> memcontrol: hook up vmpressure to socket pressure"). A quick look at
> the commit log and the code suggests that this is used all over the
> socket and tcp code to throttles the memory consumption of the
> networking layer if we are under pressure.
> 
> However, for proactive reclaim like memory.reclaim, the target is to
> probe the memcg for cold memory. Reclaiming such memory should not
> have a visible effect on the workload performance. I don't think that
> any network throttling side effects are correct here.

Please describe the user visible effects of this change. IIUC this is
changing the vmpressure semantic for pre-existing users (v1 when setting
the hard limit for example) and it really should be explained why
this is good for them after those years. I do not see any actual bug
being described explicitly so please make sure this is all properly
documented.

-- 
Michal Hocko
SUSE Labs