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Date:   Fri, 16 Dec 2022 09:40:34 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Wei Xu <weixugc@google.com>
Cc:     Mina Almasry <almasrymina@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Huang, Ying" <ying.huang@intel.com>, Tejun Heo <tj@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Yosry Ahmed <yosryahmed@google.com>, fvdl@google.com,
        bagasdotme@gmail.com, cgroups@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v3] mm: Add nodes= arg to memory.reclaim
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On Thu 15-12-22 09:58:12, Wei Xu wrote:
> On Wed, Dec 14, 2022 at 2:23 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Tue 13-12-22 11:29:45, Mina Almasry wrote:
> > > On Tue, Dec 13, 2022 at 6:03 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Tue 13-12-22 14:30:40, Johannes Weiner wrote:
> > > > > On Tue, Dec 13, 2022 at 02:30:57PM +0800, Huang, Ying wrote:
> > > > [...]
> > > > > > After these discussion, I think the solution maybe use different
> > > > > > interfaces for "proactive demote" and "proactive reclaim".  That is,
> > > > > > reconsider "memory.demote".  In this way, we will always uncharge the
> > > > > > cgroup for "memory.reclaim".  This avoid the possible confusion there.
> > > > > > And, because demotion is considered aging, we don't need to disable
> > > > > > demotion for "memory.reclaim", just don't count it.
> > > > >
> > > > > Hm, so in summary:
> > > > >
> > > > > 1) memory.reclaim would demote and reclaim like today, but it would
> > > > >    change to only count reclaimed pages against the goal.
> > > > >
> > > > > 2) memory.demote would only demote.
> > > > >
> > >
> > > If the above 2 points are agreeable then yes, this sounds good to me
> > > and does address our use case.
> > >
> > > > >    a) What if the demotion targets are full? Would it reclaim or fail?
> > > > >
> > >
> > > Wei will chime in if he disagrees, but I think we _require_ that it
> > > fails, not falls back to reclaim. The interface is asking for
> > > demotion, and is called memory.demote. For such an interface to fall
> > > back to reclaim would be very confusing to userspace and may trigger
> > > reclaim on a high priority job that we want to shield from proactive
> > > reclaim.
> >
> > But what should happen if the immediate demotion target is full but
> > lower tiers are still usable. Should the first one demote before
> > allowing to demote from the top tier?
> 
> In that case, the demotion will fall back to the lower tiers.  See
> node_get_allowed_targets() and establish_demotion_targets()..

I am not talking about an implicit behavior that we do not want to cast
into interface. If we want to allow a fine grained control over demotion
then the implementation shouldn't rely on the current behavior.

[...]
> > Is there any strong reason for that? We do not have any interface to
> > control NUMA balancing from userspace. Why cannot we use the interface
> > for that purpose?
> 
> A demotion interface such as memory.demote will trigger the demotion
> code path in the kernel, which depends on multiple memory tiers.

Demotion is just a fancy name of a directed migration. There is no realy
dependency on the HW nor the technology.

> I think what you are getting is a more general page migration
> interface for memcg, which will need both source and target nodes as
> arguments. I think this can be a great idea.  It should be able to
> support our demotion use cases as well.

yes.

-- 
Michal Hocko
SUSE Labs