[PATCH v5] mm/vmscan: more restrictive condition for retry in do_try_to_free_pages

[Yisheng Xie <ysxie@foxmail.com>]

From: Yisheng Xie <xieyisheng1@huawei.com>

By reviewing code, I find that when enter do_try_to_free_pages, the
may_thrash is always clear, and it will retry shrink zones to tap
cgroup's reserves memory by setting may_thrash when the former
shrink_zones reclaim nothing.

However, when memcg is disabled or on legacy hierarchy, or there do not
have any memcg protected by low limit, it should not do this useless
retry at all, for we do not have any cgroup's reserves memory to tap,
and we have already done hard work but made no progress, which as Michal
pointed out in former version, we are trying hard to control the retry
logical of page alloctor, and the current additional round of reclaim is
just lame.

Therefore, to avoid this unneeded retrying and make code more readable,
we remove the may_thrash field in scan_control, instead, introduce
memcg_low_reclaim and memcg_low_skipped, and only retry when
memcg_low_skipped, by setting memcg_low_reclaim.

Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Michal Hocko <mhocko@kernel.org>
Suggested-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
v5:
 - remove may_thrash field in scan_control, and introduce mem_cgroup_reclaim
   and memcg_low_skipped to make code more readable. - Johannes

v4:
 - add a new field in scan_control named memcg_low_protection to check whether
   there have any memcg protected by low limit. - Michal

v3:
 - rename function may_thrash() to mem_cgroup_thrashed() to avoid confusing.

v2:
 - more restrictive condition for retry of shrink_zones (restricting
   cgroup_disabled=memory boot option and cgroup legacy hierarchy) - Shakeel

 - add a stub function may_thrash() to avoid compile error or warning.

 - rename subject from "donot retry shrink zones when memcg is disable"
   to "more restrictive condition for retry in do_try_to_free_pages"

Any comment is more than welcome!

Hi, Andrew,
Could you please help to drop the v4, thank you so much.

Thanks
Yisheng Xie

 mm/vmscan.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bc8031e..d214212 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -97,8 +97,13 @@ struct scan_control {
 	/* Can pages be swapped as part of reclaim? */
 	unsigned int may_swap:1;
 
-	/* Can cgroups be reclaimed below their normal consumption range? */
-	unsigned int may_thrash:1;
+	/*
+	 * Cgroups are not reclaimed below their configured memory.low,
+	 * unless we threaten to OOM. If any cgroups are skipped due to
+	 * memory.low and nothing was reclaimed, go back for memory.low.
+	 */
+	unsigned int memcg_low_reclaim:1;
+	unsigned int memcg_low_skipped:1;
 
 	unsigned int hibernation_mode:1;
 
@@ -2557,8 +2562,10 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 			unsigned long scanned;
 
 			if (mem_cgroup_low(root, memcg)) {
-				if (!sc->may_thrash)
+				if (!sc->memcg_low_reclaim) {
+					sc->memcg_low_skipped = 1;
 					continue;
+				}
 				mem_cgroup_events(memcg, MEMCG_LOW, 1);
 			}
 
@@ -2808,9 +2815,10 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 		return 1;
 
 	/* Untapped cgroup reserves?  Don't OOM, retry. */
-	if (!sc->may_thrash) {
+	if (sc->memcg_low_skipped) {
 		sc->priority = initial_priority;
-		sc->may_thrash = 1;
+		sc->memcg_low_reclaim = 1;
+		sc->memcg_low_skipped = 0;
 		goto retry;
 	}
 
-- 
1.9.1

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