Commit 631b6e08 authored May 03, 2017 by Mel Gorman Committed by Linus Torvalds May 03, 2017

mm, vmscan: only clear pgdat congested/dirty/writeback state when balanced

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

4.11.0-rc1 4.11.0-rc1 4.11.0-rc1
vanilla fixcheck-v2 clear-v2
Amean p50-Read 21670074.18 ( 0.00%) 20464344.18 ( 5.56%) 19786774.76 ( 8.69%)
Amean p95-Read 25456267.64 ( 0.00%) 25721423.64 ( -1.04%) 24101956.27 ( 5.32%)
Amean p99-Read 29369064.73 ( 0.00%) 30174230.76 ( -2.74%) 27691872.71 ( 5.71%)
Amean p50-Write 1390.30 ( 0.00%) 1395.28 ( -0.36%) 1011.91 ( 27.22%)
Amean p95-Write 412901.57 ( 0.00%) 37737.74 ( 90.86%) 34874.98 ( 91.55%)
Amean p99-Write 6668722.09 ( 0.00%) 666489.04 ( 90.01%) 575449.60 ( 91.37%)
Amean p50-Allocation 78714.31 ( 0.00%) 86286.22 ( -9.62%) 84246.26 ( -7.03%)
Amean p95-Allocation 175533.51 ( 0.00%) 351812.27 (-100.42%) 400058.43 (-127.91%)
Amean p99-Allocation 247003.02 ( 0.00%) 6291171.56 (-2447.00%) 10905600.00 (-4315.17%)

Read latency is improved, write latency is mostly improved but
allocation latency is regressed. 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.

Link: http://lkml.kernel.org/r/20170309075657.25121-3-mgorman@techsingularity.net

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shantanu Goel <sgoel01@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 333b0a45

mm/vmscan.c

+11 −9

Original line number	Diff line number	Diff line
		@@ -3056,17 +3056,17 @@ 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.
		@@ -3103,9 +3103,11 @@ 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;
		}

Admin message