Re: [PATCH 1/3] mm, vmscan: fix zone balance check in prepare_kswapd_sleep

[Vlastimil Babka]

On 03/09/2017 08:56 AM, Mel Gorman wrote:
> From: Shantanu Goel <sgoel01@xxxxxxxxx>
> 
> 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.
> 
> It was first reported against 4.9.7 that 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.
> For the machine that originally motivated 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 49% 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).
> 
>                                          4.11.0-rc1            4.11.0-rc1
>                                             vanilla           fixcheck-v2
> Amean    p50-Read             21670074.18 (  0.00%) 20464344.18 (  5.56%)
> Amean    p95-Read             25456267.64 (  0.00%) 25721423.64 ( -1.04%)
> Amean    p99-Read             29369064.73 (  0.00%) 30174230.76 ( -2.74%)
> Amean    p50-Write                1390.30 (  0.00%)     1395.28 ( -0.36%)
> Amean    p95-Write              412901.57 (  0.00%)    37737.74 ( 90.86%)
> Amean    p99-Write             6668722.09 (  0.00%)   666489.04 ( 90.01%)
> Amean    p50-Allocation          78714.31 (  0.00%)    86286.22 ( -9.62%)
> Amean    p95-Allocation         175533.51 (  0.00%)   351812.27 (-100.42%)
> Amean    p99-Allocation         247003.02 (  0.00%)  6291171.56 (-2447.00%)
> 
> Of greater concern is that the patch causes swapping and page writes
> from kswapd context rose from 0 pages to 4189753 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 <sgoel01@xxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>