[PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

Mel Gorman
2021-12-02T13:19:04Z

Mike Galbraith, Alexey Avramov and Darrick Wong all reported similar
problems due to reclaim throttling for excessive lengths of time.
In Alexey's case, a memory hog that should go OOM quickly stalls for
several minutes before stalling. In Mike and Darrick's cases, a small
memcg environment stalled excessively even though the system had enough
memory overall.

Commit 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being
made") introduced the problem although commit a19594ca4a8b ("mm/vmscan:
increase the timeout if page reclaim is not making progress") made it
worse. Systems at or near an OOM state that cannot be recovered must
reach OOM quickly and memcg should kill tasks if a memcg is near OOM.

To address this, only stall for the first zone in the zonelist, reduce
the timeout to 1 tick for VMSCAN_THROTTLE_NOPROGRESS and only stall if
the scan control nr_reclaimed is 0, kswapd is still active and there were
excessive pages pending for writeback. If kswapd has stopped reclaiming due
to excessive failures, do not stall at all so that OOM triggers relatively
quickly. Similarly, if an LRU is simply congested, only lightly throttle
similar to NOPROGRESS.

Alexey's original case was the most straight forward

for i in {1..3}; do tail /dev/zero; done

On vanilla 5.16-rc1, this test stalled heavily, after the patch the test
completes in a few seconds similar to 5.15.

Alexey's second test case added watching a youtube video while tail runs
10 times. On 5.15, playback only jitters slightly, 5.16-rc1 stalls a lot
with lots of frames missing and numerous audio glitches. With this patch
applies, the video plays similarly to 5.15.

Link: https://lore.kernel.org/r/99e779783d6c7fce96448a3402061b9dc1b3b602.camel@gmx.de
Link: https://lore.kernel.org/r/20211124011954.7cab9bb4@mail.inbox.lv
Link: https://lore.kernel.org/r/20211022144651.19914-1-mgorman@techsingularity.net

Reported-and-tested-by: Alexey Avramov <hakavlad@inbox.lv>
Reported-and-tested-by: Mike Galbraith <efault@gmx.de>
Reported-and-tested-by: Darrick J. Wong <djwong@kernel.org>
Fixes: 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being made")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
include/linux/mmzone.h | 1 +
include/trace/events/vmscan.h | 4 ++-
mm/vmscan.c | 64 ++++++++++++++++++++++++++++++-----
3 files changed, 59 insertions(+), 10 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 58e744b78c2c..936dc0b6c226 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -277,6 +277,7 @@ enum vmscan_throttle_state {
VMSCAN_THROTTLE_WRITEBACK,
VMSCAN_THROTTLE_ISOLATED,
VMSCAN_THROTTLE_NOPROGRESS,
+ VMSCAN_THROTTLE_CONGESTED,
NR_VMSCAN_THROTTLE,
};

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index f25a6149d3ba..ca2e9009a651 100644
--- a/include/trace/events/vmscan.h
+++ b/include/trace/events/vmscan.h
@@ -30,12 +30,14 @@
#define _VMSCAN_THROTTLE_WRITEBACK (1 << VMSCAN_THROTTLE_WRITEBACK)
#define _VMSCAN_THROTTLE_ISOLATED (1 << VMSCAN_THROTTLE_ISOLATED)
#define _VMSCAN_THROTTLE_NOPROGRESS (1 << VMSCAN_THROTTLE_NOPROGRESS)
+#define _VMSCAN_THROTTLE_CONGESTED (1 << VMSCAN_THROTTLE_CONGESTED)

#define show_throttle_flags(flags) \
(flags) ? __print_flags(flags, "|", \
{_VMSCAN_THROTTLE_WRITEBACK, "VMSCAN_THROTTLE_WRITEBACK"}, \
{_VMSCAN_THROTTLE_ISOLATED, "VMSCAN_THROTTLE_ISOLATED"}, \
- {_VMSCAN_THROTTLE_NOPROGRESS, "VMSCAN_THROTTLE_NOPROGRESS"} \
+ {_VMSCAN_THROTTLE_NOPROGRESS, "VMSCAN_THROTTLE_NOPROGRESS"}, \
+ {_VMSCAN_THROTTLE_CONGESTED, "VMSCAN_THROTTLE_CONGESTED"} \
) : "VMSCAN_THROTTLE_NONE"

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fb9584641ac7..e3f2dd1e8cd9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1021,6 +1021,39 @@ static void handle_write_error(struct address_space *mapping,
unlock_page(page);
}

+bool skip_throttle_noprogress(pg_data_t *pgdat)
+{
+ int reclaimable = 0, write_pending = 0;
+ int i;
+
+ /*
+ * If kswapd is disabled, reschedule if necessary but do not
+ * throttle as the system is likely near OOM.
+ */
+ if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
+ return true;
+
+ /*
+ * If there are a lot of dirty/writeback pages then do not
+ * throttle as throttling will occur when the pages cycle
+ * towards the end of the LRU if still under writeback.
+ */
+ for (i = 0; i < MAX_NR_ZONES; i++) {
+ struct zone *zone = pgdat->node_zones + i;
+
+ if (!populated_zone(zone))
+ continue;
+
+ reclaimable += zone_reclaimable_pages(zone);
+ write_pending += zone_page_state_snapshot(zone,
+ NR_ZONE_WRITE_PENDING);
+ }
+ if (2 * write_pending <= reclaimable)
+ return true;
+
+ return false;
+}
+
void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
{
wait_queue_head_t *wqh = &pgdat->reclaim_wait[reason];
@@ -1056,8 +1089,16 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
}

break;
+ case VMSCAN_THROTTLE_CONGESTED:
+ fallthrough;
case VMSCAN_THROTTLE_NOPROGRESS:
- timeout = HZ/2;
+ if (skip_throttle_noprogress(pgdat)) {
+ cond_resched();
+ return;
+ }
+
+ timeout = 1;
+
break;
case VMSCAN_THROTTLE_ISOLATED:
timeout = HZ/50;
@@ -3321,7 +3362,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
if (!current_is_kswapd() && current_may_throttle() &&
!sc->hibernation_mode &&
test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_CONGESTED);

if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
sc))
@@ -3386,16 +3427,16 @@ static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
}

/*
- * Do not throttle kswapd on NOPROGRESS as it will throttle on
- * VMSCAN_THROTTLE_WRITEBACK if there are too many pages under
- * writeback and marked for immediate reclaim at the tail of
- * the LRU.
+ * Do not throttle kswapd or cgroup reclaim on NOPROGRESS as it will
+ * throttle on VMSCAN_THROTTLE_WRITEBACK if there are too many pages
+ * under writeback and marked for immediate reclaim at the tail of the
+ * LRU.
*/
- if (current_is_kswapd())
+ if (current_is_kswapd() || cgroup_reclaim(sc))
return;

/* Throttle if making no progress at high prioities. */
- if (sc->priority < DEF_PRIORITY - 2)
+ if (sc->priority == 1 && !sc->nr_reclaimed)
reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
}

@@ -3415,6 +3456,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
unsigned long nr_soft_scanned;
gfp_t orig_mask;
pg_data_t *last_pgdat = NULL;
+ pg_data_t *first_pgdat = NULL;

/*
* If the number of buffer_heads in the machine exceeds the maximum
@@ -3478,14 +3520,18 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
/* need some check for avoid more shrink_zone() */
}

+ if (!first_pgdat)
+ first_pgdat = zone->zone_pgdat;
+
/* See comment about same check for global reclaim above */
if (zone->zone_pgdat == last_pgdat)
continue;
last_pgdat = zone->zone_pgdat;
shrink_node(zone->zone_pgdat, sc);
- consider_reclaim_throttle(zone->zone_pgdat, sc);
}

+ consider_reclaim_throttle(first_pgdat, sc);
+
/*
* Restore to original mask to avoid the impact on the caller if we
* promoted it to __GFP_HIGHMEM.
--
2.31.1