[PATCH] sched: Only migrate tasks due to interrupts on an idle CPU if prev and target CPUs share cache

[Mel Gorman]

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 as well as any CPU or just one CPU. 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's 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 noticable
in the CPU migration statistics which are reduced by 24%.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2fe3aa853e4d..4a1f7d32ecf6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5701,7 +5701,13 @@ 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)