[X/B][PATCH] mm: do not stall register_shrinker()

[Mauricio Faria de Oliveira]

From: Minchan Kim <minchan at kernel.org>

BugLink: https://bugs.launchpad.net/bugs/1817628

Shakeel Butt reported he has observed in production systems that the job
loader gets stuck for 10s of seconds while doing a mount operation.  It
turns out that it was stuck in register_shrinker() because some
unrelated job was under memory pressure and was spending time in
shrink_slab().  Machines have a lot of shrinkers registered and jobs
under memory pressure have to traverse all of those memcg-aware
shrinkers and affect unrelated jobs which want to register their own
shrinkers.

To solve the issue, this patch simply bails out slab shrinking if it is
found that someone wants to register a shrinker in parallel.  A downside
is it could cause unfair shrinking between shrinkers.  However, it
should be rare and we can add compilcated logic if we find it's not
enough.

[akpm at linux-foundation.org: tweak code comment]
Link: http://lkml.kernel.org/r/20171115005602.GB23810@bbox
Link: http://lkml.kernel.org/r/1511481899-20335-1-git-send-email-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan at kernel.org>
Signed-off-by: Shakeel Butt <shakeelb at google.com>
Reported-by: Shakeel Butt <shakeelb at google.com>
Tested-by: Shakeel Butt <shakeelb at google.com>
Acked-by: Johannes Weiner <hannes at cmpxchg.org>
Acked-by: Michal Hocko <mhocko at suse.com>
Cc: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
Cc: Anshuman Khandual <khandual at linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(backported from commit e496612c5130567fc9d5f1969ca4b86665aa3cbb)
[mfo: refresh one context line for do_shrink_slab() arguments]
Signed-off-by: Mauricio Faria de Oliveira <mfo at canonical.com>
---
 mm/vmscan.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f53dcdccdb83..decc5f99b85c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -453,6 +453,15 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 			sc.nid = 0;
 
 		freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
+		/*
+		 * Bail out if someone want to register a new shrinker to
+		 * prevent the regsitration from being stalled for long periods
+		 * by parallel ongoing shrinking.
+		 */
+		if (rwsem_is_contended(&shrinker_rwsem)) {
+			freed = freed ? : 1;
+			break;
+		}
 	}
 
 	up_read(&shrinker_rwsem);
-- 
2.17.1