linux-kernel - Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

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Date:   Tue, 5 Apr 2022 18:07:49 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Cgroups <cgroups@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Jonathan Corbet <corbet@....net>, Yu Zhao <yuzhao@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

On Tue, Apr 5, 2022 at 5:49 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Wei Xu <weixugc@...gle.com> writes:
>
> > On Sat, Apr 2, 2022 at 1:13 AM Huang, Ying <ying.huang@...el.com> wrote:
> >>
> >> Wei Xu <weixugc@...gle.com> writes:
> >>
> >> > On Fri, Apr 1, 2022 at 6:54 AM Michal Hocko <mhocko@...e.com> wrote:
> >> >>
> >> >> On Thu 31-03-22 08:41:51, Yosry Ahmed wrote:
> >> >> > From: Shakeel Butt <shakeelb@...gle.com>
> >> >> >
> >>
> >> [snip]
> >>
> >> >> > Possible Extensions:
> >> >> > --------------------
> >> >> >
> >> >> > - This interface can be extended with an additional parameter or flags
> >> >> >   to allow specifying one or more types of memory to reclaim from (e.g.
> >> >> >   file, anon, ..).
> >> >> >
> >> >> > - The interface can also be extended with a node mask to reclaim from
> >> >> >   specific nodes. This has use cases for reclaim-based demotion in memory
> >> >> >   tiering systens.
> >> >> >
> >> >> > - A similar per-node interface can also be added to support proactive
> >> >> >   reclaim and reclaim-based demotion in systems without memcg.
> >> >> >
> >> >> > For now, let's keep things simple by adding the basic functionality.
> >> >>
> >> >> Yes, I am for the simplicity and this really looks like a bare minumum
> >> >> interface. But it is not really clear who do you want to add flags on
> >> >> top of it?
> >> >>
> >> >> I am not really sure we really need a node aware interface for memcg.
> >> >> The global reclaim interface will likely need a different node because
> >> >> we do not want to make this CONFIG_MEMCG constrained.
> >> >
> >> > A nodemask argument for memory.reclaim can be useful for memory
> >> > tiering between NUMA nodes with different performance.  Similar to
> >> > proactive reclaim, it can allow a userspace daemon to drive
> >> > memcg-based proactive demotion via the reclaim-based demotion
> >> > mechanism in the kernel.
> >>
> >> I am not sure whether nodemask is a good way for demoting pages between
> >> different types of memory.  For example, for a system with DRAM and
> >> PMEM, if specifying DRAM node in nodemask means demoting to PMEM, what
> >> is the meaning of specifying PMEM node? reclaiming to disk?
> >>
> >> In general, I have no objection to the idea in general.  But we should
> >> have a clear and consistent interface.  Per my understanding the default
> >> memcg interface is for memory, regardless of memory types.  The memory
> >> reclaiming means reduce the memory usage, regardless of memory types.
> >> We need to either extending the semantics of memory reclaiming (to
> >> include memory demoting too), or add another interface for memory
> >> demoting.
> >
> > Good point.  With the "demote pages during reclaim" patch series,
> > reclaim is already extended to demote pages as well.  For example,
> > can_reclaim_anon_pages() returns true if demotion is allowed and
> > shrink_page_list() can demote pages instead of reclaiming pages.
>
> These are in-kernel implementation, not the ABI.  So we still have
> the opportunity to define the ABI now.
>
> > Currently, demotion is disabled for memcg reclaim, which I think can
> > be relaxed and also necessary for memcg-based proactive demotion.  I'd
> > like to suggest that we extend the semantics of memory.reclaim to
> > cover memory demotion as well.  A flag can be used to enable/disable
> > the demotion behavior.
>
> If so,
>
> # echo A > memory.reclaim
>
> means
>
> a) "A" bytes memory are freed from the memcg, regardless demoting is
>    used or not.
>
> or
>
> b) "A" bytes memory are reclaimed from the memcg, some of them may be
>    freed, some of them may be just demoted from DRAM to PMEM.  The total
>    number is "A".
>
> For me, a) looks more reasonable.
>

We can use a DEMOTE flag to control the demotion behavior for
memory.reclaim.  If the flag is not set (the default), then
no_demotion of scan_control can be set to 1, similar to
reclaim_pages().

The question is then whether we want to rename memory.reclaim to
something more general.  I think this name is fine if reclaim-based
demotion is an accepted concept.