[PATCH] sched/fair: check for idle core

[Julia Lawall]

On a thread wakeup, the change [1] from runnable load average to load
average for comparing candidate cores means that recent short-running
daemons on the core where a thread ran previously can be considered to
have a higher load than the core performing the wakeup, even when the
core where the thread ran previously is currently idle.  This can
cause a thread to migrate, taking the place of some other thread that
is about to wake up, and so on.  To avoid unnecessary migrations,
extend wake_affine_idle to check whether the core where the thread
previously ran is currently idle, and if so return that core as the
target.

[1] commit 11f10e5420f6ce ("sched/fair: Use load instead of runnable
load in wakeup path")

This particularly has an impact when using passive (intel_cpufreq)
power management, where kworkers run every 0.004 seconds on all cores,
increasing the likelihood that an idle core will be considered to have
a load.

The following numbers were obtained with the benchmarking tool
hyperfine (https://github.com/sharkdp/hyperfine) on the NAS parallel
benchmarks (https://www.nas.nasa.gov/publications/npb.html).  The
tests were run on an 80-core Intel(R) Xeon(R) CPU E7-8870 v4 @
2.10GHz.  Active (intel_pstate) and passive (intel_cpufreq) power
management were used.  Times are in seconds.  All experiments use all
160 hardware threads.

	v5.9/active		v5.9+patch/active
bt.C.c	24.725724+-0.962340	23.349608+-1.607214
lu.C.x	29.105952+-4.804203	25.249052+-5.561617
sp.C.x	31.220696+-1.831335	30.227760+-2.429792
ua.C.x	26.606118+-1.767384	25.778367+-1.263850

	v5.9/passive		v5.9+patch/passive
bt.C.c	25.330360+-1.028316	23.544036+-1.020189
lu.C.x	35.872659+-4.872090	23.719295+-3.883848
sp.C.x	32.141310+-2.289541	29.125363+-0.872300
ua.C.x	29.024597+-1.667049	25.728888+-1.539772

On the smaller data sets (A and B) and on the other NAS benchmarks
there is no impact on performance.

Signed-off-by: Julia Lawall <Julia.Lawall@xxxxxxxx>

---
 kernel/sched/fair.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index aa4c6227cd6d..9b23dad883ee 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5804,6 +5804,9 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
 	if (sync && cpu_rq(this_cpu)->nr_running == 1)
 		return this_cpu;
 
+	if (available_idle_cpu(prev_cpu))
+		return prev_cpu;
+
 	return nr_cpumask_bits;
 }