kswapd is woken to reclaim a node based on a failed allocation request
from any eligible zone.  Once reclaiming in balance_pgdat(), it will
continue reclaiming until there is an eligible zone available for the
zone it was woken for.  kswapd tracks what zone it was recently woken
for in pgdat->kswapd_classzone_idx.  If it has not been woken recently,
this zone will be 0.

However, the decision on whether to sleep is made on
kswapd_classzone_idx which is 0 without a recent wakeup request and that
classzone does not account for lowmem reserves.  This allows kswapd to
sleep when a low small zone such as ZONE_DMA is balanced for a GFP_DMA
request even if a stream of allocations cannot use that zone.  While
kswapd may be woken again shortly in the near future there are two
consequences -- the pgdat bits that control congestion are cleared
prematurely and direct reclaim is more likely as kswapd slept
prematurely.

This patch flips kswapd_classzone_idx to default to MAX_NR_ZONES (an
invalid index) when there has been no recent wakeups.  If there are no
wakeups, it'll decide whether to sleep based on the highest possible
zone available (MAX_NR_ZONES - 1).  It then becomes critical that the
"pgdat balanced" decisions during reclaim and when deciding to sleep are
the same.  If there is a mismatch, kswapd can stay awake continually
trying to balance tiny zones.

simoop was used to evaluate it again.  Two of the preparation patches
regressed the workload so they are included as the second set of
results.  Otherwise this patch looks artifically excellent

                                         4.11.0-rc1            4.11.0-rc1            4.11.0-rc1
                                            vanilla              clear-v2          keepawake-v2
Amean    p50-Read             21670074.18 (  0.00%) 19786774.76 (  8.69%) 22668332.52 ( -4.61%)
Amean    p95-Read             25456267.64 (  0.00%) 24101956.27 (  5.32%) 26738688.00 ( -5.04%)
Amean    p99-Read             29369064.73 (  0.00%) 27691872.71 (  5.71%) 30991404.52 ( -5.52%)
Amean    p50-Write                1390.30 (  0.00%)     1011.91 ( 27.22%)      924.91 ( 33.47%)
Amean    p95-Write              412901.57 (  0.00%)    34874.98 ( 91.55%)     1362.62 ( 99.67%)
Amean    p99-Write             6668722.09 (  0.00%)   575449.60 ( 91.37%)    16854.04 ( 99.75%)
Amean    p50-Allocation          78714.31 (  0.00%)    84246.26 ( -7.03%)    74729.74 (  5.06%)
Amean    p95-Allocation         175533.51 (  0.00%)   400058.43 (-127.91%)   101609.74 ( 42.11%)
Amean    p99-Allocation         247003.02 (  0.00%) 10905600.00 (-4315.17%)   125765.57 ( 49.08%)

With this patch on top, write and allocation latencies are massively
improved.  The read latencies are slightly impaired but it's worth
noting that this is mostly due to the IO scheduler and not directly
related to reclaim.  The vmstats are a bit of a mix but the relevant
ones are as follows;

                            4.10.0-rc7  4.10.0-rc7  4.10.0-rc7
                          mmots-20170209 clear-v1r25keepawake-v1r25
Swap Ins                             0           0           0
Swap Outs                            0         608           0
Direct pages scanned           6910672     3132699     6357298
Kswapd pages scanned          57036946    82488665    56986286
Kswapd pages reclaimed        55993488    63474329    55939113
Direct pages reclaimed         6905990     2964843     6352115
Kswapd efficiency                  98%         76%         98%
Kswapd velocity              12494.375   17597.507   12488.065
Direct efficiency                  99%         94%         99%
Direct velocity               1513.835     668.306    1393.148
Page writes by reclaim           0.000 4410243.000       0.000
Page writes file                     0     4409635           0
Page writes anon                     0         608           0
Page reclaim immediate         1036792    14175203     1042571

                            4.11.0-rc1  4.11.0-rc1  4.11.0-rc1
                               vanilla  clear-v2  keepawake-v2
Swap Ins                             0          12           0
Swap Outs                            0         838           0
Direct pages scanned           6579706     3237270     6256811
Kswapd pages scanned          61853702    79961486    54837791
Kswapd pages reclaimed        60768764    60755788    53849586
Direct pages reclaimed         6579055     2987453     6256151
Kswapd efficiency                  98%         75%         98%
Page writes by reclaim           0.000 4389496.000       0.000
Page writes file                     0     4388658           0
Page writes anon                     0         838           0
Page reclaim immediate         1073573    14473009      982507

Swap-outs are equivalent to baseline.

Direct reclaim is reduced but not eliminated.  It's worth noting that
there are two periods of direct reclaim for this workload.  The first is
when it switches from preparing the files for the actual test itself.
It's a lot of file IO followed by a lot of allocs that reclaims heavily
for a brief window.  While direct reclaim is lower with clear-v2, it is
due to kswapd scanning aggressively and trying to reclaim the world
which is not the right thing to do.  With the patches applied, there is
still direct reclaim but the phase change from "creating work files" to
starting multiple threads that allocate a lot of anonymous memory faster
than kswapd can reclaim.

Scanning/reclaim efficiency is restored by this patch.

Page writes from reclaim context are back at 0 which is ideal.

Pages immediately reclaimed after IO completes is slightly improved but
it is expected this will vary slightly.

On UMA, there is almost no change so this is not expected to be a
universal win.

[mgorman@suse.de: fix ->kswapd_classzone_idx initialization]
  Link: http://lkml.kernel.org/r/20170406174538.5msrznj6nt6qpbx5@suse.de
Link: http://lkml.kernel.org/r/20170309075657.25121-4-mgorman@techsingularity.net


Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
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>