On Sun, Feb 05, 2017 at 12:46:20PM +0000, Shantanu Goel wrote: > > 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 > This should be in the changelog of the patch itself and the patch should be inline instead of being an attachment. > > Thanks, > Shantanu > From 46f2e4b02ac263bf50d69cdab3bcbd7bcdea7415 Mon Sep 17 00:00:00 2001 > From: Shantanu Goel <sgoel01@xxxxxxxxx> > Date: Sat, 4 Feb 2017 19:07:53 -0500 > Subject: [PATCH] vmscan: fix zone balance check in prepare_kswapd_sleep > > 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. > > Signed-off-by: Shantanu Goel <sgoel01@xxxxxxxxx> I don't recall specifically why I made that change but I've no objections to the patch so; Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> However, note that there is a slight risk that kswapd will sleep for a short interval early due to a very small zone such as ZONE_DMA. If this is a general problem then it'll manifest as less kswapd reclaim and more direct reclaim. If it turns out this is an issue then a revert will not be the right fix. Instead, all the checks for zone_balance will need to account for the only balanced zone being a tiny percentage of memory in the node. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
