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

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Date:   Wed, 29 Jun 2022 18:07:00 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Michal Hocko <mhocko@...e.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 Mon, Jun 27, 2022 at 10:04 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Mon, Jun 27, 2022 at 5:31 AM Michal Hocko <mhocko@...e.com> wrote:
> >
[...]
> >
> > I can see clear arguments for memory.reclaim opt out for vmpressure
> > because we have established that this is not a measure to express a
> > memory pressure on the cgroup.
> >
> > Max/High are less clear to me, TBH. I do understand reasoning for PSI
> > exclusion because considering the calling process to be stalled and
> > non-productive is misleading. It just does its work so in a way it is
> > a productive time in the end. For the vmpressure, which measures how
> > hard/easy it is to reclaim memory why this should special for this
> > particular reclaim?
> >
> > Again, an explanation of the effect on the socket pressure could give a
> > better picture. Say that I somebody reduces the limit (hard/high) and it
> > takes quite some effort to shrink the consumption down. Should the
> > networking layer react to that in any way or should it wait for the
> > active allocation during that process to find that out?
>
> I am out of my depth here. Any answer on my side would be purely
> speculation at this point. Shakeel, can you help us here or tag some
> networking people?

So, the effect of returning true from mem_cgroup_under_socket_pressure() are:

1. Reducing send and receive buffers of the current socket.
2. May drop packets on the rx path.
3. May throttle current thread on the tx path.

Now regarding the behavior from the reclaim due to reducing max or
high, I think the kernel should not ignore vmpressure. Please note
that unlike PSI which is associated with the current process,
vmpressure is associated with the target memcg. So, any reclaim on
that memcg due to real shortage of memory should not be ignored. That
reclaim can be global reclaim or limit reclaim of ancestor or itself
or reclaim due to lowering the limit of ancestor or itself.