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From:   Wei Xu <weixugc@google.com>
Date:   Sat, 2 Apr 2022 23:56:19 -0700
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Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Yosry Ahmed <yosryahmed@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Jonathan Corbet <corbet@lwn.net>, Yu Zhao <yuzhao@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Greg Thelen <gthelen@google.com>
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On Sat, Apr 2, 2022 at 1:13 AM Huang, Ying <ying.huang@intel.com> wrote:
>
> Wei Xu <weixugc@google.com> writes:
>
> > On Fri, Apr 1, 2022 at 6:54 AM Michal Hocko <mhocko@suse.com> wrote:
> >>
> >> On Thu 31-03-22 08:41:51, Yosry Ahmed wrote:
> >> > From: Shakeel Butt <shakeelb@google.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.

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.