The patch titled
Subject: mm: memcg: let non-unified root stats flushes help unified flushes
has been added to the -mm mm-unstable branch. Its filename is
mm-memcg-let-non-unified-root-stats-flushes-help-unified-flushes.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memcg-let-non-unified-root-stats-flushes-help-unified-flushes.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Subject: mm: memcg: let non-unified root stats flushes help unified flushes
Date: Thu, 31 Aug 2023 16:56:10 +0000
Unified flushing of memcg stats keeps track of the magnitude of pending
updates, and only allows a flush if that magnitude exceeds a threshold.
It also keeps track of the time at which ratelimited flushing should be
allowed as flush_next_time.
A non-unified flush on the root memcg has the same effect as a unified
flush, so let it help unified flushing by resetting pending updates and
kicking flush_next_time forward. Move the logic into the common
do_stats_flush() helper, and do it for all root flushes, unified or not.
There is a subtle change here, we reset stats_flush_threshold before a
flush rather than after a flush. This probably okay because:
(a) For flushers: only unified flushers check stats_flush_threshold,
and those flushers skip anyway if there is another unified flush
ongoing. Having them also skip if there is an ongoing non-unified
root flush is actually more consistent.
(b) For updaters: Resetting stats_flush_threshold early may lead to
more atomic updates of stats_flush_threshold, as we start updating it
earlier. This should not be significant in practice because we stop
updating stats_flush_threshold when it reaches the threshold anyway.
If we start early and stop early, the number of atomic updates remain
the same. The only difference is the scenario where we reset
stats_flush_threshold early, start doing atomic updates early, and
then the periodic flusher kicks in before we reach the threshold. In
this case, we will have done more atomic updates. However, since the
threshold wasn't reached, then we did not do a lot of updates anyway.
Link: https://lkml.kernel.org/r/20230831165611.2610118-4-yosryahmed@xxxxxxxxxx
Suggested-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Cc: Ivan Babrou <ivan@xxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
--- a/mm/memcontrol.c~mm-memcg-let-non-unified-root-stats-flushes-help-unified-flushes
+++ a/mm/memcontrol.c
@@ -647,6 +647,11 @@ static inline void memcg_rstat_updated(s
*/
static void do_stats_flush(struct mem_cgroup *memcg)
{
+ /* for unified flushing, root non-unified flushing can help as well */
+ if (mem_cgroup_is_root(memcg)) {
+ WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
+ atomic_set(&stats_flush_threshold, 0);
+ }
cgroup_rstat_flush(memcg->css.cgroup);
}
@@ -665,11 +670,8 @@ static void do_unified_stats_flush(void)
atomic_xchg(&stats_unified_flush_ongoing, 1))
return;
- WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
-
do_stats_flush(root_mem_cgroup);
- atomic_set(&stats_flush_threshold, 0);
atomic_set(&stats_unified_flush_ongoing, 0);
}
_
Patches currently in -mm which might be from yosryahmed@xxxxxxxxxx are
mm-memcg-properly-name-and-document-unified-stats-flushing.patch
mm-memcg-add-a-helper-for-non-unified-stats-flushing.patch
mm-memcg-let-non-unified-root-stats-flushes-help-unified-flushes.patch
mm-memcg-use-non-unified-stats-flushing-for-userspace-reads.patch
