From: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
Subject: mm: vmscan: do not pass reclaimed slab to vmpressure
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. In this use
case, The number of critical vmpressure events received is around 50%
less on 4.4 than 3.18. 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. The difference in reclaim behaviour is because of
lesser number of kills and kills happening late, resulting in more
swapping and page cache reclaim.
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
The lowmemorykiller example above is just for indicating the difference
in vmpressure events between 4.4 and 3.18.
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. 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()")
Link: http://lkml.kernel.org/r/1486641577-11685-2-git-send-email-vinmenon@xxxxxxxxxxxxxx
Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
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: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
diff -puN mm/vmscan.c~mm-vmscan-do-not-pass-reclaimed-slab-to-vmpressure mm/vmscan.c
--- a/mm/vmscan.c~mm-vmscan-do-not-pass-reclaimed-slab-to-vmpressure
+++ a/mm/vmscan.c
@@ -2567,16 +2567,23 @@ static bool shrink_node(pg_data_t *pgdat
shrink_slab(sc->gfp_mask, pgdat->node_id, NULL,
sc->priority);
+ /*
+ * 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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