Wei Xu <weixugc@xxxxxxxxxx> writes: > On Tue, Apr 5, 2022 at 5:49 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Wei Xu <weixugc@xxxxxxxxxx> writes: >> >> > On Sat, Apr 2, 2022 at 1:13 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> Wei Xu <weixugc@xxxxxxxxxx> writes: >> >> >> >> > On Fri, Apr 1, 2022 at 6:54 AM Michal Hocko <mhocko@xxxxxxxx> wrote: >> >> >> >> >> >> On Thu 31-03-22 08:41:51, Yosry Ahmed wrote: >> >> >> > From: Shakeel Butt <shakeelb@xxxxxxxxxx> >> >> >> > >> >> >> >> [snip] >> >> >> >> >> > Possible Extensions: >> >> >> > -------------------- >> >> >> > >> >> >> > - This interface can be extended with an additional parameter or flags >> >> >> > to allow specifying one or more types of memory to reclaim from (e.g. >> >> >> > file, anon, ..). >> >> >> > >> >> >> > - The interface can also be extended with a node mask to reclaim from >> >> >> > specific nodes. This has use cases for reclaim-based demotion in memory >> >> >> > tiering systens. >> >> >> > >> >> >> > - A similar per-node interface can also be added to support proactive >> >> >> > reclaim and reclaim-based demotion in systems without memcg. >> >> >> > >> >> >> > For now, let's keep things simple by adding the basic functionality. >> >> >> >> >> >> Yes, I am for the simplicity and this really looks like a bare minumum >> >> >> interface. But it is not really clear who do you want to add flags on >> >> >> top of it? >> >> >> >> >> >> I am not really sure we really need a node aware interface for memcg. >> >> >> The global reclaim interface will likely need a different node because >> >> >> we do not want to make this CONFIG_MEMCG constrained. >> >> > >> >> > A nodemask argument for memory.reclaim can be useful for memory >> >> > tiering between NUMA nodes with different performance. Similar to >> >> > proactive reclaim, it can allow a userspace daemon to drive >> >> > memcg-based proactive demotion via the reclaim-based demotion >> >> > mechanism in the kernel. >> >> >> >> I am not sure whether nodemask is a good way for demoting pages between >> >> different types of memory. For example, for a system with DRAM and >> >> PMEM, if specifying DRAM node in nodemask means demoting to PMEM, what >> >> is the meaning of specifying PMEM node? reclaiming to disk? >> >> >> >> In general, I have no objection to the idea in general. But we should >> >> have a clear and consistent interface. Per my understanding the default >> >> memcg interface is for memory, regardless of memory types. The memory >> >> reclaiming means reduce the memory usage, regardless of memory types. >> >> We need to either extending the semantics of memory reclaiming (to >> >> include memory demoting too), or add another interface for memory >> >> demoting. >> > >> > Good point. With the "demote pages during reclaim" patch series, >> > reclaim is already extended to demote pages as well. For example, >> > can_reclaim_anon_pages() returns true if demotion is allowed and >> > shrink_page_list() can demote pages instead of reclaiming pages. >> >> These are in-kernel implementation, not the ABI. So we still have >> the opportunity to define the ABI now. >> >> > Currently, demotion is disabled for memcg reclaim, which I think can >> > be relaxed and also necessary for memcg-based proactive demotion. I'd >> > like to suggest that we extend the semantics of memory.reclaim to >> > cover memory demotion as well. A flag can be used to enable/disable >> > the demotion behavior. >> >> If so, >> >> # echo A > memory.reclaim >> >> means >> >> a) "A" bytes memory are freed from the memcg, regardless demoting is >> used or not. >> >> or >> >> b) "A" bytes memory are reclaimed from the memcg, some of them may be >> freed, some of them may be just demoted from DRAM to PMEM. The total >> number is "A". >> >> For me, a) looks more reasonable. >> > > We can use a DEMOTE flag to control the demotion behavior for > memory.reclaim. If the flag is not set (the default), then > no_demotion of scan_control can be set to 1, similar to > reclaim_pages(). If we have to use a flag to control the behavior, I think it's better to have a separate interface (e.g. memory.demote). But do we really need b)? > The question is then whether we want to rename memory.reclaim to > something more general. I think this name is fine if reclaim-based > demotion is an accepted concept. Best Regards, Huang, Ying
