A previous patch series ([1] currently in mm-stable) changed most atomic rstat flushing contexts to become non-atomic. This was done to avoid an expensive operation that scales with # cgroups and # cpus to happen with irqs disabled and scheduling not permitted. There were two remaining atomic flushing contexts after that series. This series tries to eliminate them as well, eliminating atomic rstat flushing completely. The two remaining atomic flushing contexts are: (a) wb_over_bg_thresh()->mem_cgroup_wb_stats() (b) mem_cgroup_threshold()->mem_cgroup_usage() For (a), flushing needs to be atomic as wb_writeback() calls wb_over_bg_thresh() with a spinlock held. However, it seems like the call to wb_over_bg_thresh() doesn't need to be protected by that spinlock, so this series proposes a refactoring that moves the call outside the lock criticial section and makes the stats flushing in mem_cgroup_wb_stats() non-atomic. For (b), flushing needs to be atomic as mem_cgroup_threshold() is called with irqs disabled. We only flush the stats when calculating the root usage, as it is approximated as the sum of some memcg stats (file, anon, and optionally swap) instead of the conventional page counter. This series proposes changing this calculation to use the global stats instead, eliminating the need for a memcg stat flush. After these 2 contexts are eliminated, we no longer need mem_cgroup_flush_stats_atomic() or cgroup_rstat_flush_atomic(). We can remove them and simplify the code. [1] https://lore.kernel.org/linux-mm/20230330191801.1967435-1-yosryahmed@xxxxxxxxxx/ RFC -> v1: - Collected R-b's and A-b's (Thanks everyone!). - Rebased onto mm-stable. - Cosmetic changes to commit logs. RFC: https://lore.kernel.org/linux-mm/20230403220337.443510-1-yosryahmed@xxxxxxxxxx/ Yosry Ahmed (5): writeback: move wb_over_bg_thresh() call outside lock section memcg: flush stats non-atomically in mem_cgroup_wb_stats() memcg: calculate root usage from global state memcg: remove mem_cgroup_flush_stats_atomic() cgroup: remove cgroup_rstat_flush_atomic() fs/fs-writeback.c | 16 +++++++---- include/linux/cgroup.h | 1 - include/linux/memcontrol.h | 5 ---- kernel/cgroup/rstat.c | 26 ++++-------------- mm/memcontrol.c | 54 ++++++++------------------------------ 5 files changed, 27 insertions(+), 75 deletions(-) -- 2.40.0.634.g4ca3ef3211-goog
