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