[RFC PATCH 6/7] workingset: memcg: sleep when flushing stats in workingset_refault()

Yosry Ahmed
2023-03-23T04:01:22Z

In workingset_refault(), we call mem_cgroup_flush_stats_delayed() to
flush stats within an RCU read section and with sleeping disallowed.
Move the call to mem_cgroup_flush_stats_delayed() above the RCU read
section and allow sleeping to avoid unnecessarily performing a lot of
work without sleeping.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
---

A lot of code paths call into workingset_refault(), so I am not
generally sure at all whether it's okay to sleep in all contexts or not.
Feedback here would be very helpful.

---
mm/workingset.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 042eabbb43f6..410bc6684ea7 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -406,6 +406,8 @@ void workingset_refault(struct folio *folio, void *shadow)
unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
eviction <<= bucket_order;

+ /* Flush stats (and potentially sleep) before holding RCU read lock */
+ mem_cgroup_flush_stats_delayed(true);
rcu_read_lock();
/*
* Look up the memcg associated with the stored ID. It might
@@ -461,9 +463,6 @@ void workingset_refault(struct folio *folio, void *shadow)
lruvec = mem_cgroup_lruvec(memcg, pgdat);

mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
-
- /* Do not sleep with RCU lock held */
- mem_cgroup_flush_stats_delayed(false);
/*
* Compare the distance to the existing workingset size. We
* don't activate pages that couldn't stay resident even if
--
2.40.0.rc1.284.g88254d51c5-goog