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(). 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.
