[RFC PATCH v1 0/1] select idle cpu from idle cpumask in sched domain

[Aubrey Li]

I'm writting to see if it makes sense to track idle cpus in a shared cpumask
in sched domain, then a task wakes up it can select idle cpu from this cpumask
instead of scanning all the cpus in the last level cache domain, especially
when the system is heavily loaded, the scanning cost could be significantly
reduced. The price is that the atomic cpumask ops are added to the idle entry
and exit paths.

I tested the following benchmarks on a x86 4 socket system with 24 cores per
socket and 2 hyperthreads per core, total 192 CPUs:

uperf throughput: netperf workload, tcp_nodelay, r/w size = 90

  threads	baseline-avg	%std	patch-avg	%std
  96		1		1.24	0.98		2.76
  144		1		1.13	1.35		4.01
  192		1		0.58	1.67		3.25
  240		1		2.49	1.68		3.55

hackbench: process mode, 100000 loops, 40 file descriptors per group

  group		baseline-avg	%std	patch-avg	%std
  2(80)		1		12.05	0.97		9.88
  3(120)	1		12.48	0.95		11.62
  4(160)	1		13.83	0.97		13.22
  5(200)	1		2.76	1.01		2.94	 

schbench: 99th percentile latency, 16 workers per message thread

  mthread	baseline-avg	%std	patch-avg	%std
  6(96)		1		1.24	0.993		1.73
  9(144)	1		0.38	0.998		0.39
  12(192)	1		1.58	0.995		1.64
  15(240)	1		51.71	0.606		37.41

sysbench mysql throughput: read/write, table size = 10,000,000

  thread	baseline-avg	%std	patch-avg	%std
  96		1		1.77	1.015		1.71
  144		1		3.39	0.998		4.05
  192		1		2.88	1.002		2.81
  240		1		2.07	1.011		2.09

kbuild: kexec reboot every time

  baseline-avg	patch-avg
  1		1

Any suggestions are highly appreciated!

Thanks,
-Aubrey

Aubrey Li (1):
  sched/fair: select idle cpu from idle cpumask in sched domain

 include/linux/sched/topology.h | 13 +++++++++++++
 kernel/sched/fair.c            |  4 +++-
 kernel/sched/topology.c        |  2 +-
 3 files changed, 17 insertions(+), 2 deletions(-)

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2.25.1