On Tue 13-12-22 11:29:45, Mina Almasry wrote: > On Tue, Dec 13, 2022 at 6:03 AM Michal Hocko <mhocko@xxxxxxxx> 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. > > 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? -- Michal Hocko SUSE Labs
