Date:   Fri, 16 Dec 2022 11:02:22 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mina Almasry <almasrymina@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Jonathan Corbet <corbet@....net>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Yosry Ahmed <yosryahmed@...gle.com>, weixugc@...gle.com,
        fvdl@...gle.com, bagasdotme@...il.com, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v3] mm: Add nodes= arg to memory.reclaim

Michal Hocko <mhocko@...e.com> writes:

> On Thu 15-12-22 13:50:14, Huang, Ying wrote:
>> Michal Hocko <mhocko@...e.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@...e.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). 

Yes.  We should consider the potential requirements.

>> >> > 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.

Yes.

Best Regards,
Huang, Ying

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