Re: [PATCH] vmscan: fix zone balance check in prepare_kswapd_sleep

[Mel Gorman <mgorman@techsingularity.net>]

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@yahoo.com>
> 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@yahoo.com>

I don't recall specifically why I made that change but I've no objections
to the patch so;

Acked-by: Mel Gorman <mgorman@techsingularity.net>

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

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