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

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Date:   Thu, 7 Apr 2022 21:10:33 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        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 Thu, Apr 7, 2022 at 8:08 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Wei Xu <weixugc@...gle.com> writes:
>
> > On Thu, Apr 7, 2022 at 4:11 PM Tim Chen <tim.c.chen@...ux.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.
>
> I think that it may be sufficient to make tier an attribute of "node".
> Some thing like,
>
> /sys/devices/system/node/nodeX/memory_tier
>

This works. If we want additional information about each tier, we can
then add a tier-specific subtree.

In addition, it would be good to also expose the demotion target nodes
(node_demotion[]) via sysfs, e.g.:

/sys/devices/system/node/nodeX/demotion_path

which returns node_demotion[X].

> Best Regards,
> Huang, Ying
>
> > 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
> >>
>