From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751635AbdBOJXR (ORCPT ); Wed, 15 Feb 2017 04:23:17 -0500 Received: from outbound-smtp04.blacknight.com ([81.17.249.35]:34853 "EHLO outbound-smtp04.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751305AbdBOJWv (ORCPT ); Wed, 15 Feb 2017 04:22:51 -0500 From: Mel Gorman To: Andrew Morton Cc: Shantanu Goel , Chris Mason , Johannes Weiner , Vlastimil Babka , LKML , Linux-MM , Mel Gorman Subject: [PATCH 1/3] mm, vmscan: fix zone balance check in prepare_kswapd_sleep Date: Wed, 15 Feb 2017 09:22:45 +0000 Message-Id: <20170215092247.15989-2-mgorman@techsingularity.net> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170215092247.15989-1-mgorman@techsingularity.net> References: <20170215092247.15989-1-mgorman@techsingularity.net> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Shantanu Goel The check in prepare_kswapd_sleep needs to match the one in balance_pgdat since the latter will return as soon as any one of the zones in the classzone is above the watermark. This is specially important for higher order allocations since balance_pgdat will typically reset the order to zero relying on compaction to create the higher order pages. Without this patch, prepare_kswapd_sleep fails to wake up kcompactd since the zone balance check fails. On 4.9.7 kswapd is failing to wake up kcompactd due to a mismatch in the zone balance check between balance_pgdat() and prepare_kswapd_sleep(). balance_pgdat() returns as soon as a single zone satisfies the allocation but prepare_kswapd_sleep() requires all zones to do +the same. This causes prepare_kswapd_sleep() to never succeed except in the order == 0 case and consequently, wakeup_kcompactd() is never called. On my machine prior to apply this patch, the state of compaction from /proc/vmstat looked this way after a day and a half +of uptime: compact_migrate_scanned 240496 compact_free_scanned 76238632 compact_isolated 123472 compact_stall 1791 compact_fail 29 compact_success 1762 compact_daemon_wake 0 After applying the patch and about 10 hours of uptime the state looks like this: compact_migrate_scanned 59927299 compact_free_scanned 2021075136 compact_isolated 640926 compact_stall 4 compact_fail 2 compact_success 2 compact_daemon_wake 5160 Further notes from Mel that motivated him to pick this patch up and resend it; It was observed for the simoop workload (pressures the VM similar to HADOOP) that kswapd was failing to keep ahead of direct reclaim. The investigation noted that there was a need to rationalise kswapd decisions to reclaim with kswapd decisions to sleep. With this patch on a 2-socket box, there was a 43% reduction in direct reclaim scanning. However, the impact otherwise is extremely negative. Kswapd reclaim efficiency dropped from 98% to 76%. simoop has three latency-related metrics for read, write and allocation (an anonymous mmap and fault). 4.10.0-rc7 4.10.0-rc7 mmots-20170209 fixcheck-v1 Amean p50-Read 22325202.49 ( 0.00%) 20026926.55 ( 10.29%) Amean p95-Read 26102988.80 ( 0.00%) 27023360.00 ( -3.53%) Amean p99-Read 30935176.53 ( 0.00%) 30994432.00 ( -0.19%) Amean p50-Write 976.44 ( 0.00%) 1905.28 (-95.12%) Amean p95-Write 15471.29 ( 0.00%) 36210.09 (-134.05%) Amean p99-Write 35108.62 ( 0.00%) 479494.96 (-1265.75%) Amean p50-Allocation 76382.61 ( 0.00%) 87603.20 (-14.69%) Amean p95-Allocation 127777.39 ( 0.00%) 244491.38 (-91.34%) Amean p99-Allocation 187937.39 ( 0.00%) 1745237.33 (-828.63%) There are also more allocation stalls. One of the largest impacts was due to pages written back from kswapd context rising from 0 pages to 4516642 pages during the hour the workload ran for. By and large, the patch has very bad behaviour but easily missed as the impact on a UMA machine is negligible. This patch is included with the data in case a bisection leads to this area. This patch is also a pre-requisite for the rest of the series. Signed-off-by: Shantanu Goel Signed-off-by: Mel Gorman --- mm/vmscan.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 26c3b405ef34..92fc66bd52bc 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3140,11 +3140,11 @@ 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)) - return false; + if (zone_balanced(zone, order, classzone_idx)) + return true; } - return true; + return false; } /* -- 2.11.0