On Sat 19-02-22 09:49:40, Suren Baghdasaryan wrote: > When page allocation in direct reclaim path fails, the system will > make one attempt to shrink per-cpu page lists and free pages from > high alloc reserves. Draining per-cpu pages into buddy allocator can > be a very slow operation because it's done using workqueues and the > task in direct reclaim waits for all of them to finish before > proceeding. Currently this time is not accounted as psi memory stall. > > While testing mobile devices under extreme memory pressure, when > allocations are failing during direct reclaim, we notices that psi > events which would be expected in such conditions were not triggered. > After profiling these cases it was determined that the reason for > missing psi events was that a big chunk of time spent in direct > reclaim is not accounted as memory stall, therefore psi would not > reach the levels at which an event is generated. Further investigation > revealed that the bulk of that unaccounted time was spent inside > drain_all_pages call. It would be cool to have some numbers here. > Annotate drain_all_pages and unreserve_highatomic_pageblock during > page allocation failure in the direct reclaim path so that delays > caused by these calls are accounted as memory stall. If the draining is too slow and dependent on the current CPU/WQ contention then we should address that. The original intention was that having a dedicated WQ with WQ_MEM_RECLAIM would help to isolate the operation from the rest of WQ activity. Maybe we need to fine tune mm_percpu_wq. If that doesn't help then we should revise the WQ model and use something else. Memory reclaim shouldn't really get stuck behind other unrelated work. -- Michal Hocko SUSE Labs