On Fri, Dec 9, 2022 at 1:16 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Fri 09-12-22 08:41:47, Wei Xu wrote: > > On Fri, Dec 9, 2022 at 12:08 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Thu 08-12-22 16:59:36, Wei Xu wrote: > > > [...] > > > > > What I really mean is to add demotion nodes to the nodemask along with > > > > > the set of nodes you want to reclaim from. To me that sounds like a > > > > > more natural interface allowing for all sorts of usecases: > > > > > - free up demotion targets (only specify demotion nodes in the mask) > > > > > - control where to demote (e.g. select specific demotion target(s)) > > > > > - do not demote at all (skip demotion nodes from the node mask) > > > > > > > > For clarification, do you mean to add another argument (e.g. > > > > demotion_nodes) in addition to the "nodes" argument? > > > > > > No, nodes=mask argument should control the domain where the memory > > > reclaim should happen. That includes both aging and the reclaim. If the > > > mask doesn't contain any lower tier node then no demotion will happen. > > > If only a subset of lower tiers are specified then only those could be > > > used for the demotion process. Or put it otherwise, the nodemask is not > > > only used to filter out zonelists during reclaim it also restricts > > > migration targets. > > > > > > Is this more clear now? > > > > In that case, how can we request demotion only from toptier nodes > > (without counting any reclaimed bytes from other nodes), which is our > > memory tiering use case? > > I am not sure I follow. Could you be more specific please? In our memory tiering use case, we would like to proactively free up memory on top-tier nodes by demoting cold pages to lower-tier nodes. This is to create enough free top-tier memory for new allocations and promotions. How many pages and how often to demote from top-tier nodes can depend on a number of factors (e.g. the amount of free top-tier memory, the amount of cold pages, the bandwidth pressure on lower-tier, the task tolerance of slower memory on performance) and are controlled by the userspace policies. Because the purpose of such proactive demotions is to free up top-tier memory, not to lower the amount of memory charged to the memcg, we'd like that memory.reclaim can demote the specified amount of bytes from the given top-tier nodes. If we have to also provide the lower-tier nodes to memory.reclaim to allow demotions, the kernel can reclaim from the lower-tier nodes in the same memory.reclaim request. We then won't be able to control the amount of bytes to be demoted from top-tier nodes. > > Besides, when both toptier and demotion nodes are specified, the > > demoted pages should only be counted as aging and not be counted > > towards the requested bytes of try_to_free_mem_cgroup_pages(), which > > is what this patch tries to address. > > This should be addressed by > http://lkml.kernel.org/r/Y5B1K5zAE0PkjFZx@xxxxxxxxxxxxxx, no? > -- > Michal Hocko > SUSE Labs
