linux-kernel - Re: [PATCH v3] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

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Date:   Sat, 10 Dec 2022 00:01:28 -0800
From:   Wei Xu <weixugc@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mina Almasry <almasrymina@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Huang Ying <ying.huang@...el.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Yosry Ahmed <yosryahmed@...gle.com>, fvdl@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

On Fri, Dec 9, 2022 at 1:16 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Fri 09-12-22 08:41:47, Wei Xu wrote:
> > On Fri, Dec 9, 2022 at 12:08 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Thu 08-12-22 16:59:36, Wei Xu wrote:
> > > [...]
> > > > > What I really mean is to add demotion nodes to the nodemask along with
> > > > > the set of nodes you want to reclaim from. To me that sounds like a
> > > > > more natural interface allowing for all sorts of usecases:
> > > > > - free up demotion targets (only specify demotion nodes in the mask)
> > > > > - control where to demote (e.g. select specific demotion target(s))
> > > > > - do not demote at all (skip demotion nodes from the node mask)
> > > >
> > > > For clarification, do you mean to add another argument (e.g.
> > > > demotion_nodes) in addition to the "nodes" argument?
> > >
> > > No, nodes=mask argument should control the domain where the memory
> > > reclaim should happen. That includes both aging and the reclaim. If the
> > > mask doesn't contain any lower tier node then no demotion will happen.
> > > If only a subset of lower tiers are specified then only those could be
> > > used for the demotion process. Or put it otherwise, the nodemask is not
> > > only used to filter out zonelists during reclaim it also restricts
> > > migration targets.
> > >
> > > Is this more clear now?
> >
> > In that case, how can we request demotion only from toptier nodes
> > (without counting any reclaimed bytes from other nodes),  which is our
> > memory tiering use case?
>
> I am not sure I follow. Could you be more specific please?

In our memory tiering use case, we would like to proactively free up
memory on top-tier nodes by demoting cold pages to lower-tier nodes.
This is to create enough free top-tier memory for new allocations and
promotions.  How many pages and how often to demote from top-tier
nodes can depend on a number of factors (e.g. the amount of free
top-tier memory, the amount of cold pages, the bandwidth pressure on
lower-tier, the task tolerance of slower memory on performance) and
are controlled by the userspace policies.

Because the purpose of such proactive demotions is to free up top-tier
memory, not to lower the amount of memory charged to the memcg, we'd
like that memory.reclaim can demote the specified amount of bytes from
the given top-tier nodes.  If we have to also provide the lower-tier
nodes to memory.reclaim to allow demotions, the kernel can reclaim
from the lower-tier nodes in the same memory.reclaim request. We then
won't be able to control the amount of bytes to be demoted from
top-tier nodes.

> > Besides, when both toptier and demotion nodes are specified, the
> > demoted pages should only be counted as aging and not be counted
> > towards the requested bytes of try_to_free_mem_cgroup_pages(), which
> > is what this patch tries to address.
>
> This should be addressed by
> http://lkml.kernel.org/r/Y5B1K5zAE0PkjFZx@dhcp22.suse.cz, no?
> --
> Michal Hocko
> SUSE Labs