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. 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.
