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Date:   Thu, 15 Dec 2022 10:21:25 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     Mina Almasry <almasrymina@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        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>,
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        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 13:50:14, Huang, Ying wrote:
> Michal Hocko <mhocko@suse.com> writes:
> 
> > 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?
> >  
> >> > > 3) Would memory.reclaim and memory.demote still need nodemasks?
> >> 
> >> memory.demote will need a nodemask, for sure. Today the nodemask would
> >> be useful if there is a specific node in the top tier that is
> >> overloaded and we want to reduce the pressure by demoting. In the
> >> future there will be N tiers and the nodemask says which tier to
> >> demote from.
> >
> > OK, so what is the exact semantic of the node mask. Does it control
> > where to demote from or to or both?
> >
> >> I don't think memory.reclaim would need a nodemask anymore? At least I
> >> no longer see the use for it for us.
> >> 
> >> > >    Would
> >> > >    they return -EINVAL if a) memory.reclaim gets passed only toptier
> >> > >    nodes or b) memory.demote gets passed any lasttier nodes?
> >> >
> >> 
> >> Honestly it would be great if memory.reclaim can force reclaim from a
> >> top tier nodes. It breaks the aginig pipeline, yes, but if the user is
> >> specifically asking for that because they decided in their usecase
> >> it's a good idea then the kernel should comply IMO. Not a strict
> >> requirement for us. Wei will chime in if he disagrees.
> >
> > That would require a nodemask to say which nodes to reclaim, no? The
> > default behavior should be in line with what standard memory reclaim
> > does. If the demotion is a part of that process so should be
> > memory.reclaim part of it. If we want to have a finer control then a
> > nodemask is really a must and then the nodemaks should constrain both
> > agining and reclaim.
> >
> >> memory.demote returning -EINVAL for lasttier nodes makes sense to me.
> >> 
> >> > I would also add
> >> > 4) Do we want to allow to control the demotion path (e.g. which node to
> >> >    demote from and to) and how to achieve that?
> >> 
> >> We care deeply about specifying which node to demote _from_. That
> >> would be some node that is approaching pressure and we're looking for
> >> proactive saving from. So far I haven't seen any reason to control
> >> which nodes to demote _to_. The kernel deciding that based on the
> >> aging pipeline and the node distances sounds good to me. Obviously
> >> someone else may find that useful.
> >
> > Please keep in mind that the interface should be really prepared for
> > future extensions so try to abstract from your immediate usecases.
> 
> I see two requirements here, one is to control the demotion source, that
> is, which nodes to free memory.  The other is to control the demotion
> path.  I think that we can use two different parameters for them, for
> example, "from=<demotion source nodes>" and "to=<demotion target
> nodes>".  In most cases we don't need to control the demotion path.
> Because in current implementation, the nodes in the lower tiers in the
> same socket (local nodes) will be preferred.  I think that this is
> the desired behavior in most cases.

Even if the demotion path is not really required at the moment we should
keep in mind future potential extensions. E.g. when a userspace based
balancing is to be implemented because the default behavior cannot
capture userspace policies (one example would be enforcing a
prioritization of containers when some container's demoted pages would
need to be demoted further to free up a space for a different
workload). 
 
> >> > 5) Is the demotion api restricted to multi-tier systems or any numa
> >> >    configuration allowed as well?
> >> >
> >> 
> >> demotion will of course not work on single tiered systems. The
> >> interface may return some failure on such systems or not be available
> >> at all.
> >
> > 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? 
> 
> Do you mean to demote the cold pages from the specified source nodes to
> the specified target nodes in different sockets?  We don't do that to
> avoid loop in the demotion path.  If we prevent the target nodes from
> demoting cold pages to the source nodes at the same time, it seems
> doable.

Loops could be avoid by properly specifying from and to nodes if this is
going to be a fine grained interface to control demotion.
-- 
Michal Hocko
SUSE Labs