[PATCH 2/2 v5] mm: vmscan: do not pass reclaimed slab to vmpressure

[Vinayak Menon]

During global reclaim, the nr_reclaimed passed to vmpressure includes the
pages reclaimed from slab.  But the corresponding scanned slab pages is
not passed. There is an impact to the vmpressure values because of this.
While moving from kernel version 3.18 to 4.4, a difference is seen
in the vmpressure values for the same workload resulting in a different
behaviour of the vmpressure consumer. One such case is of a vmpressure
based lowmemorykiller. It is observed that the vmpressure events are
received late and less in number resulting in tasks not being killed at
the right time. The following numbers show the impact on reclaim activity
due to the change in behaviour of lowmemorykiller on a 4GB device. The test
launches a number of apps in sequence and repeats it multiple times.

                      v4.4           v3.18
pgpgin                163016456      145617236
pgpgout               4366220        4188004
workingset_refault    29857868       26781854
workingset_activate   6293946        5634625
pswpin                1327601        1133912
pswpout               3593842        3229602
pgalloc_dma           99520618       94402970
pgalloc_normal        104046854      98124798
pgfree                203772640      192600737
pgmajfault            2126962        1851836
pgsteal_kswapd_dma    19732899       18039462
pgsteal_kswapd_normal 19945336       17977706
pgsteal_direct_dma    206757         131376
pgsteal_direct_normal 236783         138247
pageoutrun            116622         108370
allocstall            7220           4684
compact_stall         931            856

This is a regression introduced by commit 6b4f7799c6a5 ("mm: vmscan:
invoke slab shrinkers from shrink_zone()").

So do not consider reclaimed slab pages for vmpressure calculation. The
reclaimed pages from slab can be excluded because the freeing of a page
by slab shrinking depends on each slab's object population, making the
cost model (i.e. scan:free) different from that of LRU.  Also, not every
shrinker accounts the pages it reclaims. But ideally the pages reclaimed
from slab should be passed to vmpressure, otherwise higher vmpressure
levels can be triggered even when there is a reclaim progress. But
accounting only the reclaimed slab pages without the scanned, and adding
something which does not fit into the cost model just adds noise to the
vmpressure values.

Fixes: 6b4f7799c6a5 ("mm: vmscan: invoke slab shrinkers from shrink_zone()")
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Cc: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
Cc: Shiraz Hashim <shashim@xxxxxxxxxxxxxx>
Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
---
v5: Modifying the changelog and reordering the patches
    as per Michal's suggestion

 mm/vmscan.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 947ab6f..8969f8e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2594,16 +2594,23 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 				    sc->nr_scanned - nr_scanned,
 				    node_lru_pages);
 
+		/*
+		 * Record the subtree's reclaim efficiency. The reclaimed
+		 * pages from slab is excluded here because the corresponding
+		 * scanned pages is not accounted. Moreover, freeing a page
+		 * by slab shrinking depends on each slab's object population,
+		 * making the cost model (i.e. scan:free) different from that
+		 * of LRU.
+		 */
+		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
+			   sc->nr_scanned - nr_scanned,
+			   sc->nr_reclaimed - nr_reclaimed);
+
 		if (reclaim_state) {
 			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
 			reclaim_state->reclaimed_slab = 0;
 		}
 
-		/* Record the subtree's reclaim efficiency */
-		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
-			   sc->nr_scanned - nr_scanned,
-			   sc->nr_reclaimed - nr_reclaimed);
-
 		if (sc->nr_reclaimed - nr_reclaimed)
 			reclaimable = true;
 
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