sched/fair: Only immediately migrate tasks due to interrupts if prev and target CPUs share cache

If waking from an idle CPU due to an interrupt then it's possible that
the waker task will be pulled to wake on the current CPU. Unfortunately,
depending on the type of interrupt and IRQ configuration, there may not
be a strong relationship between the CPU an interrupt was delivered on
and the CPU a task was running on. For example, the interrupts could all
be delivered to CPUs on one particular node due to the machine topology
or IRQ affinity configuration. Another example is an interrupt for an IO
completion which can be delivered to any CPU where there is no guarantee
the data is either cache hot or even local.

This patch was motivated by the observation that an IO workload was
being pulled cross-node on a frequent basis when IO completed.  From a
wakeup latency perspective, it's still useful to know that an idle CPU is
immediately available for use but lets only consider an automatic migration
if the CPUs share cache to limit damage due to NUMA migrations. Migrations
may still occur if wake_affine_weight determines it's appropriate.

These are the throughput results for dbench running on ext4 comparing
4.15-rc3 and this patch on a 2-socket machine where interrupts due to IO
completions can happen on any CPU.

                          4.15.0-rc3             4.15.0-rc3
                             vanilla            lessmigrate
Hmean     1        854.64 (   0.00%)      865.01 (   1.21%)
Hmean     2       1229.60 (   0.00%)     1274.44 (   3.65%)
Hmean     4       1591.81 (   0.00%)     1628.08 (   2.28%)
Hmean     8       1845.04 (   0.00%)     1831.80 (  -0.72%)
Hmean     16      2038.61 (   0.00%)     2091.44 (   2.59%)
Hmean     32      2327.19 (   0.00%)     2430.29 (   4.43%)
Hmean     64      2570.61 (   0.00%)     2568.54 (  -0.08%)
Hmean     128     2481.89 (   0.00%)     2499.28 (   0.70%)
Stddev    1         14.31 (   0.00%)        5.35 (  62.65%)
Stddev    2         21.29 (   0.00%)       11.09 (  47.92%)
Stddev    4          7.22 (   0.00%)        6.80 (   5.92%)
Stddev    8         26.70 (   0.00%)        9.41 (  64.76%)
Stddev    16        22.40 (   0.00%)       20.01 (  10.70%)
Stddev    32        45.13 (   0.00%)       44.74 (   0.85%)
Stddev    64        93.10 (   0.00%)       93.18 (  -0.09%)
Stddev    128      184.28 (   0.00%)      177.85 (   3.49%)

Note the small increase in throughput for low thread counts but also
note that the standard deviation for each sample during the test run is
lower. The throughput figures for dbench can be misleading so the benchmark
is actually modified to time the latency of the processing of one load
file with many samples taken. The difference in latency is

                           4.15.0-rc3             4.15.0-rc3
                              vanilla            lessmigrate
Amean      1         21.71 (   0.00%)       21.47 (   1.08%)
Amean      2         30.89 (   0.00%)       29.58 (   4.26%)
Amean      4         47.54 (   0.00%)       46.61 (   1.97%)
Amean      8         82.71 (   0.00%)       82.81 (  -0.12%)
Amean      16       149.45 (   0.00%)      145.01 (   2.97%)
Amean      32       265.49 (   0.00%)      248.43 (   6.42%)
Amean      64       463.23 (   0.00%)      463.55 (  -0.07%)
Amean      128      933.97 (   0.00%)      935.50 (  -0.16%)
Stddev     1          1.58 (   0.00%)        1.54 (   2.26%)
Stddev     2          2.84 (   0.00%)        2.95 (  -4.15%)
Stddev     4          6.78 (   0.00%)        6.85 (  -0.99%)
Stddev     8         16.85 (   0.00%)       16.37 (   2.85%)
Stddev     16        41.59 (   0.00%)       41.04 (   1.32%)
Stddev     32       111.05 (   0.00%)      105.11 (   5.35%)
Stddev     64       285.94 (   0.00%)      288.01 (  -0.72%)
Stddev     128      803.39 (   0.00%)      809.73 (  -0.79%)

It's a small improvement which is not surprising given that migrations that
migrate to a different node as not that common. However, it is noticeable
in the CPU migration statistics which are reduced by 24%.

There was a query for v1 of this patch about NAS so here are the results
for C-class using MPI for parallelisation on the same machine

nas-mpi
                      4.15.0-rc3             4.15.0-rc3
                         vanilla                  noirq
Time cg.C       24.25 (   0.00%)       23.17 (   4.45%)
Time ep.C        8.22 (   0.00%)        8.29 (  -0.85%)
Time ft.C       22.67 (   0.00%)       20.34 (  10.28%)
Time is.C        1.42 (   0.00%)        1.47 (  -3.52%)
Time lu.C       55.62 (   0.00%)       54.81 (   1.46%)
Time mg.C        7.93 (   0.00%)        7.91 (   0.25%)

          4.15.0-rc3  4.15.0-rc3
             vanilla  noirq-v1r1
User         3799.96     3748.34
System        672.10      626.15
Elapsed        91.91       79.49

lu.C sees a small gain, ft.C a large gain and ep.C and is.C see small
regressions but in terms of absolute time, the difference is small and
likely within run-to-run variance. System CPU usage is slightly reduced.

schbench from Facebook was also requested. This is a bit of a mixed bag but
it's important to note that this workload should not be heavily impacted
by wakeups from interrupt context.

                                 4.15.0-rc3             4.15.0-rc3
                                    vanilla             noirq-v1r1
Lat 50.00th-qrtle-1        41.00 (   0.00%)       41.00 (   0.00%)
Lat 75.00th-qrtle-1        42.00 (   0.00%)       42.00 (   0.00%)
Lat 90.00th-qrtle-1        43.00 (   0.00%)       44.00 (  -2.33%)
Lat 95.00th-qrtle-1        44.00 (   0.00%)       46.00 (  -4.55%)
Lat 99.00th-qrtle-1        57.00 (   0.00%)       58.00 (  -1.75%)
Lat 99.50th-qrtle-1        59.00 (   0.00%)       59.00 (   0.00%)
Lat 99.90th-qrtle-1        67.00 (   0.00%)       78.00 ( -16.42%)
Lat 50.00th-qrtle-2        40.00 (   0.00%)       51.00 ( -27.50%)
Lat 75.00th-qrtle-2        45.00 (   0.00%)       56.00 ( -24.44%)
Lat 90.00th-qrtle-2        53.00 (   0.00%)       59.00 ( -11.32%)
Lat 95.00th-qrtle-2        57.00 (   0.00%)       61.00 (  -7.02%)
Lat 99.00th-qrtle-2        67.00 (   0.00%)       71.00 (  -5.97%)
Lat 99.50th-qrtle-2        69.00 (   0.00%)       74.00 (  -7.25%)
Lat 99.90th-qrtle-2        83.00 (   0.00%)       77.00 (   7.23%)
Lat 50.00th-qrtle-4        51.00 (   0.00%)       51.00 (   0.00%)
Lat 75.00th-qrtle-4        57.00 (   0.00%)       56.00 (   1.75%)
Lat 90.00th-qrtle-4        60.00 (   0.00%)       59.00 (   1.67%)
Lat 95.00th-qrtle-4        62.00 (   0.00%)       62.00 (   0.00%)
Lat 99.00th-qrtle-4        73.00 (   0.00%)       72.00 (   1.37%)
Lat 99.50th-qrtle-4        76.00 (   0.00%)       74.00 (   2.63%)
Lat 99.90th-qrtle-4        85.00 (   0.00%)       78.00 (   8.24%)
Lat 50.00th-qrtle-8        54.00 (   0.00%)       58.00 (  -7.41%)
Lat 75.00th-qrtle-8        59.00 (   0.00%)       62.00 (  -5.08%)
Lat 90.00th-qrtle-8        65.00 (   0.00%)       66.00 (  -1.54%)
Lat 95.00th-qrtle-8        67.00 (   0.00%)       70.00 (  -4.48%)
Lat 99.00th-qrtle-8        78.00 (   0.00%)       79.00 (  -1.28%)
Lat 99.50th-qrtle-8        81.00 (   0.00%)       80.00 (   1.23%)
Lat 99.90th-qrtle-8       116.00 (   0.00%)       83.00 (  28.45%)
Lat 50.00th-qrtle-16       65.00 (   0.00%)       64.00 (   1.54%)
Lat 75.00th-qrtle-16       77.00 (   0.00%)       71.00 (   7.79%)
Lat 90.00th-qrtle-16       83.00 (   0.00%)       82.00 (   1.20%)
Lat 95.00th-qrtle-16       87.00 (   0.00%)       87.00 (   0.00%)
Lat 99.00th-qrtle-16       95.00 (   0.00%)       96.00 (  -1.05%)
Lat 99.50th-qrtle-16       99.00 (   0.00%)      103.00 (  -4.04%)
Lat 99.90th-qrtle-16      104.00 (   0.00%)      122.00 ( -17.31%)
Lat 50.00th-qrtle-32       71.00 (   0.00%)       73.00 (  -2.82%)
Lat 75.00th-qrtle-32       91.00 (   0.00%)       92.00 (  -1.10%)
Lat 90.00th-qrtle-32      108.00 (   0.00%)      107.00 (   0.93%)
Lat 95.00th-qrtle-32      118.00 (   0.00%)      115.00 (   2.54%)
Lat 99.00th-qrtle-32      134.00 (   0.00%)      129.00 (   3.73%)
Lat 99.50th-qrtle-32      138.00 (   0.00%)      133.00 (   3.62%)
Lat 99.90th-qrtle-32      149.00 (   0.00%)      146.00 (   2.01%)
Lat 50.00th-qrtle-39       83.00 (   0.00%)       81.00 (   2.41%)
Lat 75.00th-qrtle-39      105.00 (   0.00%)      102.00 (   2.86%)
Lat 90.00th-qrtle-39      120.00 (   0.00%)      119.00 (   0.83%)
Lat 95.00th-qrtle-39      129.00 (   0.00%)      128.00 (   0.78%)
Lat 99.00th-qrtle-39      153.00 (   0.00%)      149.00 (   2.61%)
Lat 99.50th-qrtle-39      166.00 (   0.00%)      156.00 (   6.02%)
Lat 99.90th-qrtle-39    12304.00 (   0.00%)    12848.00 (  -4.42%)

When heavily loaded (e.g. 99.50th-qrtle-39 indicates 39 threads), there
are small gains in many cases. Otherwise it depends on the quartile used
where it can be bad -- e.g. 75.00th-qrtle-2. However, even these results
are probably a co-incidence. For this workload, much depends on what node
the threads get placed on and their relative locality and not wakeups from
interrupt context. A larger component on how it behaves would be automatic
NUMA balancing where a fault incurred to measure locality would be a much
larger contributer to latency than the wakeup path.

This is the results from an almost identical machine that happened to run
the same test.  They only differ in terms of storage which is irrelevant
for this test.

                                 4.15.0-rc3             4.15.0-rc3
                                    vanilla             noirq-v1r1
Lat 50.00th-qrtle-1        41.00 (   0.00%)       41.00 (   0.00%)
Lat 75.00th-qrtle-1        42.00 (   0.00%)       42.00 (   0.00%)
Lat 90.00th-qrtle-1        44.00 (   0.00%)       43.00 (   2.27%)
Lat 95.00th-qrtle-1        53.00 (   0.00%)       45.00 (  15.09%)
Lat 99.00th-qrtle-1        59.00 (   0.00%)       58.00 (   1.69%)
Lat 99.50th-qrtle-1        60.00 (   0.00%)       59.00 (   1.67%)
Lat 99.90th-qrtle-1        86.00 (   0.00%)       61.00 (  29.07%)
Lat 50.00th-qrtle-2        52.00 (   0.00%)       41.00 (  21.15%)
Lat 75.00th-qrtle-2        57.00 (   0.00%)       46.00 (  19.30%)
Lat 90.00th-qrtle-2        60.00 (   0.00%)       53.00 (  11.67%)
Lat 95.00th-qrtle-2        62.00 (   0.00%)       57.00 (   8.06%)
Lat 99.00th-qrtle-2        73.00 (   0.00%)       68.00 (   6.85%)
Lat 99.50th-qrtle-2        74.00 (   0.00%)       71.00 (   4.05%)
Lat 99.90th-qrtle-2        90.00 (   0.00%)       75.00 (  16.67%)
Lat 50.00th-qrtle-4        57.00 (   0.00%)       52.00 (   8.77%)
Lat 75.00th-qrtle-4        60.00 (   0.00%)       58.00 (   3.33%)
Lat 90.00th-qrtle-4        62.00 (   0.00%)       62.00 (   0.00%)
Lat 95.00th-qrtle-4        65.00 (   0.00%)       65.00 (   0.00%)
Lat 99.00th-qrtle-4        76.00 (   0.00%)       75.00 (   1.32%)
Lat 99.50th-qrtle-4        77.00 (   0.00%)       77.00 (   0.00%)
Lat 99.90th-qrtle-4        87.00 (   0.00%)       81.00 (   6.90%)
Lat 50.00th-qrtle-8        59.00 (   0.00%)       57.00 (   3.39%)
Lat 75.00th-qrtle-8        63.00 (   0.00%)       62.00 (   1.59%)
Lat 90.00th-qrtle-8        66.00 (   0.00%)       67.00 (  -1.52%)
Lat 95.00th-qrtle-8        68.00 (   0.00%)       70.00 (  -2.94%)
Lat 99.00th-qrtle-8        79.00 (   0.00%)       80.00 (  -1.27%)
Lat 99.50th-qrtle-8        80.00 (   0.00%)       84.00 (  -5.00%)
Lat 99.90th-qrtle-8        84.00 (   0.00%)       90.00 (  -7.14%)
Lat 50.00th-qrtle-16       65.00 (   0.00%)       65.00 (   0.00%)
Lat 75.00th-qrtle-16       77.00 (   0.00%)       75.00 (   2.60%)
Lat 90.00th-qrtle-16       84.00 (   0.00%)       83.00 (   1.19%)
Lat 95.00th-qrtle-16       88.00 (   0.00%)       87.00 (   1.14%)
Lat 99.00th-qrtle-16       97.00 (   0.00%)       96.00 (   1.03%)
Lat 99.50th-qrtle-16      100.00 (   0.00%)      104.00 (  -4.00%)
Lat 99.90th-qrtle-16      110.00 (   0.00%)      126.00 ( -14.55%)
Lat 50.00th-qrtle-32       70.00 (   0.00%)       71.00 (  -1.43%)
Lat 75.00th-qrtle-32       92.00 (   0.00%)       94.00 (  -2.17%)
Lat 90.00th-qrtle-32      110.00 (   0.00%)      110.00 (   0.00%)
Lat 95.00th-qrtle-32      121.00 (   0.00%)      118.00 (   2.48%)
Lat 99.00th-qrtle-32      135.00 (   0.00%)      137.00 (  -1.48%)
Lat 99.50th-qrtle-32      140.00 (   0.00%)      146.00 (  -4.29%)
Lat 99.90th-qrtle-32      150.00 (   0.00%)      160.00 (  -6.67%)
Lat 50.00th-qrtle-39       80.00 (   0.00%)       71.00 (  11.25%)
Lat 75.00th-qrtle-39      102.00 (   0.00%)       91.00 (  10.78%)
Lat 90.00th-qrtle-39      118.00 (   0.00%)      108.00 (   8.47%)
Lat 95.00th-qrtle-39      128.00 (   0.00%)      117.00 (   8.59%)
Lat 99.00th-qrtle-39      149.00 (   0.00%)      133.00 (  10.74%)
Lat 99.50th-qrtle-39      160.00 (   0.00%)      139.00 (  13.12%)
Lat 99.90th-qrtle-39    13808.00 (   0.00%)     4920.00 (  64.37%)

Despite being nearly identical, it showed a variety of major gains so
I'm not convinced that heavy emphasis should be placed on this particular
workload in terms of evaluating this particular patch. Further evidence of
this is the fact that testing on a UMA machine showed small gains/losses
even though the patch should be a no-op on UMA.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171219085947.13136-2-mgorman@techsingularity.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -5687,8 +5687,8 @@ static int wake_wide(struct task_struct *p)
  * soonest. For the purpose of speed we only consider the waking and previous
  * CPU.
  *
- * wake_affine_idle() - only considers 'now', it check if the waking CPU is (or
- *			will be) idle.
+ * wake_affine_idle() - only considers 'now', it check if the waking CPU is
+ *			cache-affine and is (or	will be) idle.
  *
  * wake_affine_weight() - considers the weight to reflect the average
  *			  scheduling latency of the CPUs. This seems to work
@@ -5699,7 +5699,13 @@ static bool
 wake_affine_idle(struct sched_domain *sd, struct task_struct *p,
 		 int this_cpu, int prev_cpu, int sync)
 {
-	if (idle_cpu(this_cpu))
+	/*
+	 * If this_cpu is idle, it implies the wakeup is from interrupt
+	 * context. Only allow the move if cache is shared. Otherwise an
+	 * interrupt intensive workload could force all tasks onto one
+	 * node depending on the IO topology or IRQ affinity settings.
+	 */
+	if (idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu))
 		return true;
 
 	if (sync && cpu_rq(this_cpu)->nr_running == 1)