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From:   Wei Xu <weixugc@google.com>
Date:   Thu, 7 Apr 2022 19:10:20 -0700
Message-ID: <CAAPL-u-oF5HQ26w1czNCmA5VadXOfC54GbGpSppva86YEkefyA@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 4:11 PM Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> On Thu, 2022-04-07 at 15:12 -0700, Wei Xu wrote:
>
> >
> > (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.
> >
>
> We do need some kind of userspace visible tiering representation.
> Will be nice if I can tell the memory type, nodemask of nodes in tier Y with
>
> cat memory.tier_Y
>
>
> > 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.
>
> Just from the perspective of memory.reclaim and memory.demote, I think
> they could work with nodemask.  For ease of management,
> some kind of abstraction of tier information like nodemask, memory type
> and expected performance should be readily accessible by user space.
>

I agree.  The tier information should be provided at the system level.
One suggestion is to have a new directory "/sys/devices/system/tier/"
for tiers, e.g.:

/sys/devices/system/tier/tier0/memlist: all memory nodes in tier 0.
/sys/devices/system/tier/tier1/memlist: all memory nodes in tier 1.

We can discuss this tier representation in a new thread.

> Tim
>
> >
> > 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
>