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From:   Wei Xu <weixugc@google.com>
Date:   Thu, 7 Apr 2022 15:12:19 -0700
Message-ID: <CAAPL-u_aAbDOmATSA8ZvjnfBk_7EoXvLoh0etM0fB0aY1845VQ@mail.gmail.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     "Huang, Ying" <ying.huang@intel.com>,
        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 <cgroups@vger.kernel.org>,
        "open list:DOCUMENTATION" <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 Thu, Apr 7, 2022 at 2:26 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> On Wed, 2022-04-06 at 10:49 +0800, Huang, Ying wrote:
> >
> > > > 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().
> >
> > If we have to use a flag to control the behavior, I think it's better to
> > have a separate interface (e.g. memory.demote).  But do we really need b)?
> >
> > > 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.
> >
>
> memory.demote will work for 2 level of memory tiers.  But when we have 3 level
> of memory (e.g. high bandwidth memory, DRAM and PMEM),
> it gets ambiguous again of wheter we sould demote from high bandwidth memory
> or DRAM.
>
> Will something like this be more general?
>
> echo X > memory_[dram,pmem,hbm].reclaim
>
> So echo X > memory_dram.reclaim
> means that we want to free up X bytes from DRAM for the mem cgroup.
>
> echo demote > memory_dram.reclaim_policy
>
> This means that we prefer demotion for reclaim instead
> of swapping to disk.
>

(resending in plain-text, sorry).

memory.demote can work with any level of memory tiers if a nodemask
argument (or a tier argument if there is a more-explicitly defined,
userspace visible tiering representation) is provided.  The semantics
can be to demote X bytes from these nodes to their next tier.

memory_dram/memory_pmem assumes the hardware for a particular memory
tier, which is undesirable.  For example, it is entirely possible that
a slow memory tier is implemented by a lower-cost/lower-performance
DDR device connected via CXL.mem, not by PMEM.  It is better for this
interface to speak in either the NUMA node abstraction or a new tier
abstraction.

It is also desirable to make this interface stateless, i.e. not to
require the setting of memory_dram.reclaim_policy.  Any policy can be
specified as arguments to the request itself and should only affect
that particular request.

Wei