Previously we invalidated all iterators up to the memcg root, but this is overkill: we can check if currently dying memcg is stored in iterator's 'last_visited' and invalidate only those unlucky iterators.
https://jira.sw.ru/browse/PSBM-123655 Signed-off-by: Konstantin Khorenko <[email protected]> --- mm/memcontrol.c | 48 ++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 38 insertions(+), 10 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 6cde4dbe3c7b..d60dc768f762 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -192,8 +192,8 @@ struct mem_cgroup_stat2_cpu { struct mem_cgroup_reclaim_iter { /* - * last scanned hierarchy member. Valid only if last_dead_count - * matches memcg->dead_count of the hierarchy root group. + * Last scanned hierarchy member. + * If stored memcg is destroyed, the field is wiped. */ struct mem_cgroup *last_visited; unsigned long last_dead_count; @@ -1576,6 +1576,28 @@ static void mem_cgroup_iter_invalidate(struct mem_cgroup *root, int zone, node, i; unsigned seq; + /* + * When a group in the hierarchy below root is destroyed, + * the hierarchy iterator can no longer be trusted iif + * iter->last_visited contains the cgroup being destroyed. + * Check if we get this unlikely case and invalidate the iterator + * if so. + * + * Note, only "break-ed" iterators can store iter->last_visited + * == dead_memcg because normally 'last_visited' is assigned + * under 'last_visited_lock' in mem_cgroup_iter_update() and + * 'new_position' is under css_tryget() there (ref inc-ed in + * __mem_cgroup_iter_next()) and thus css will not be offlined. + * + * mem_cgroup_iter_break() in its turn puts memcg's css but does + * not wipe it from iter->last_visited. + * + * Q: Why dying memcg (dead_memcg) cannot get into + * iter->last_visited a bit later after we wipe it here? + * A: Because up to the moment of the current function execution + * css_tryget() is guaranteed to fail on 'dead_memcg'. + */ + for_each_node(node) { for (zone = 0; zone < MAX_NR_ZONES; zone++) { mz = mem_cgroup_zoneinfo(root, node, zone); @@ -1584,10 +1606,17 @@ static void mem_cgroup_iter_invalidate(struct mem_cgroup *root, iter = &mz->reclaim_iter[i]; lock = &iter->last_visited_lock; - write_seqlock(lock); - pos = iter->last_visited; - iter->last_visited = NULL; - write_sequnlock(lock); + do { + seq = read_seqbegin(lock); + pos = READ_ONCE(iter->last_visited); + } while (read_seqretry(lock, seq)); + + if (pos == dead_memcg) { + write_seqlock(lock); + pos = iter->last_visited; + if (pos == dead_memcg) + iter->last_visited = NULL; + write_sequnlock(lock); } } } @@ -1648,10 +1677,9 @@ static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter, int sequence) { /* - * We store the sequence count from the time @last_visited was - * loaded successfully instead of rereading it here so that we - * don't lose destruction events in between. We could have - * raced with the destruction of @new_position after all. + * The position saved in 'last_visited' is always valid. + * If the stored corresponding cgroup is destroyed, + * 'last_visited' is NULLed. */ write_seqlock(&iter->last_visited_lock); iter->last_visited = new_position; -- 2.24.3 _______________________________________________ Devel mailing list [email protected] https://lists.openvz.org/mailman/listinfo/devel
