Most memcg flushing contexts using "unified" flushing, where only one flusher is allowed at a time (others skip), and all flushers need to flush the entire tree. This works well with high concurrency, which mostly comes from in-kernel flushers (e.g. reclaim, refault, ..). For userspace reads, unified flushing leads to non-deterministic stats staleness and reading cost. This series clarifies and documents the differences between unified and non-unified flushing (patches 1 & 2), then opts userspace reads out of unified flushing (patch 3). This patch series is a follow up on the discussion in [1]. That was a patch that proposed that userspace reads wait for ongoing unified flushers to complete before returning. There were concerns about the latency that this introduces to userspace reads, especially with ongoing reports of expensive stat reads even with unified flushing. Hence, this series follows a different approach, by opting userspace reads out of unified flushing completely. The cost of userspace reads are now determinstic, and depend on the size of the subtree being read. This should fix both the *sometimes* expensive reads (due to flushing the entire tree) and occasional staless (due to skipping flushing). I attempted to remove unified flushing completely, but noticed that in-kernel flushers with high concurrency (e.g. hundreds of concurrent reclaimers). This sort of concurrency is not expected from userspace reads. More details about testing and some numbers in the last patch's changelog. v2 -> v3: - Renamed stats_flush_ongoing to stats_unified_flush_ongoing in patch 1 for more clarity. - Added a mutex for flushes by userspace readers to guard the internal globla rstat lock from being hogged by userspace readers as suggested by Tejun Heo and Waiman Long. - Fixed a bug in v2, where patch 4 also opted mem_cgroup_wb_stats() out of unified flushing by mistake. mem_cgroup_wb_stats() is not for userspace reading. v2: https://lore.kernel.org/lkml/20230828233319.340712-1-yosryahmed@xxxxxxxxxx/ Yosry Ahmed (4): mm: memcg: properly name and document unified stats flushing mm: memcg: add a helper for non-unified stats flushing mm: memcg: let non-unified root stats flushes help unified flushes mm: memcg: use non-unified stats flushing for userspace reads include/linux/memcontrol.h | 8 +-- mm/memcontrol.c | 106 +++++++++++++++++++++++++++---------- mm/vmscan.c | 2 +- mm/workingset.c | 4 +- 4 files changed, 85 insertions(+), 35 deletions(-) -- 2.42.0.rc2.253.gd59a3bf2b4-goog
