[PATCH 4.14 24/75] mm: do not stall register_shrinker()

[Greg Kroah-Hartman]

From: Minchan Kim <minchan@xxxxxxxxxx>

commit e496612c5130567fc9d5f1969ca4b86665aa3cbb upstream.

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@xxxxxxxxxxxxxxxxxxxx: 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@xxxxxxxxxx
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reported-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[rkolchmeyer: Backported to 4.14: adjusted context]
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/vmscan.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -502,6 +502,15 @@ static unsigned long shrink_slab(gfp_t g
 			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);