A pgdat tracks if recent reclaim encountered too many dirty, writeback
or congested pages. The flags control whether kswapd writes pages back
from reclaim context, tags pages for immediate reclaim when IO completes,
whether processes block on wait_iff_congested and whether kswapd blocks
when too many pages marked for immediate reclaim are encountered.
The state is cleared in a check function with side-effects. With the patch
"mm, vmscan: fix zone balance check in prepare_kswapd_sleep", the timing
of when the bits get cleared changed. Due to the way the check works,
it'll clear the bits if ZONE_DMA is balanced for a GFP_DMA allocation
because it does not account for lowmem reserves properly.
For the simoop workload, kswapd is not stalling when it should due to
the premature clearing, writing pages from reclaim context like crazy and
generally being unhelpful.
This patch resets the pgdat bits related to page reclaim only when kswapd
is going to sleep. The comparison with simoop is then
4.10.0-rc7 4.10.0-rc7 4.10.0-rc7
mmots-20170209 fixcheck-v1 clear-v1
Amean p50-Read 22325202.49 ( 0.00%) 20026926.55 ( 10.29%) 19491134.58 ( 12.69%)
Amean p95-Read 26102988.80 ( 0.00%) 27023360.00 ( -3.53%) 24294195.20 ( 6.93%)
Amean p99-Read 30935176.53 ( 0.00%) 30994432.00 ( -0.19%) 30397053.16 ( 1.74%)
Amean p50-Write 976.44 ( 0.00%) 1905.28 (-95.12%) 1077.22 (-10.32%)
Amean p95-Write 15471.29 ( 0.00%) 36210.09 (-134.05%) 36419.56 (-135.40%)
Amean p99-Write 35108.62 ( 0.00%) 479494.96 (-1265.75%) 102000.36 (-190.53%)
Amean p50-Allocation 76382.61 ( 0.00%) 87603.20 (-14.69%) 87485.22 (-14.54%)
Amean p95-Allocation 127777.39 ( 0.00%) 244491.38 (-91.34%) 204588.52 (-60.11%)
Amean p99-Allocation 187937.39 ( 0.00%) 1745237.33 (-828.63%) 631657.74 (-236.10%)
Read latency is improved although write and allocation latency is
impacted. Even with the patch, kswapd is still reclaiming inefficiently,
pages are being written back from writeback context and a host of other
issues. However, given the change, it needed to be spelled out why the
side-effect was moved.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
mm/vmscan.c | 20 +++++++++++---------
1 file changed, 11 insertions(+), 9 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 92fc66bd52bc..b47b430ca7ea 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3097,17 +3097,17 @@ static bool zone_balanced(struct zone *zone, int order, int classzone_idx)
if (!zone_watermark_ok_safe(zone, order, mark, classzone_idx))
return false;
- /*
- * If any eligible zone is balanced then the node is not considered
- * to be congested or dirty
- */
- clear_bit(PGDAT_CONGESTED, &zone->zone_pgdat->flags);
- clear_bit(PGDAT_DIRTY, &zone->zone_pgdat->flags);
- clear_bit(PGDAT_WRITEBACK, &zone->zone_pgdat->flags);
-
return true;
}
+/* Clear pgdat state for congested, dirty or under writeback. */
+static void clear_pgdat_congested(pg_data_t *pgdat)
+{
+ clear_bit(PGDAT_CONGESTED, &pgdat->flags);
+ clear_bit(PGDAT_DIRTY, &pgdat->flags);
+ clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
+}
+
/*
* Prepare kswapd for sleeping. This verifies that there are no processes
* waiting in throttle_direct_reclaim() and that watermarks have been met.
@@ -3140,8 +3140,10 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
if (!managed_zone(zone))
continue;
- if (zone_balanced(zone, order, classzone_idx))
+ if (zone_balanced(zone, order, classzone_idx)) {
+ clear_pgdat_congested(pgdat);
return true;
+ }
}
return false;
--
2.11.0
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