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;
}
