On 21/10/20 16:03, Aubrey Li wrote:
> From: Aubrey Li <aubrey...@intel.com>
>
> Added idle cpumask to track idle cpus in sched domain. When a CPU
> enters idle, its corresponding bit in the idle cpumask will be set,
> and when the CPU exits idle, its bit will be cleared.
>
> When a task wakes up to select an idle cpu, scanning idle cpumask
> has low cost than scanning all the cpus in last level cache domain,
> especially when the system is heavily loaded.
>
FWIW I gave this a spin on my arm64 desktop (Ampere eMAG, 32 core). I get
some barely noticeable (AIUI not statistically significant for bench sched)
changes for 100 iterations of:
| bench | metric | mean | std | q90 |
q99 |
|------------------------------------+----------+--------+---------+--------+--------|
| hackbench --loops 5000 --groups 1 | duration | -1.07% | -2.23% | -0.88% |
-0.25% |
| hackbench --loops 5000 --groups 2 | duration | -0.79% | +30.60% | -0.49% |
-0.74% |
| hackbench --loops 5000 --groups 4 | duration | -0.54% | +6.99% | -0.21% |
-0.12% |
| perf bench sched pipe -T -l 100000 | ops/sec | +1.05% | -2.80% | -0.17% |
+0.39% |
q90 & q99 being the 90th and 99th percentile.
Base was tip/sched/core at:
d8fcb81f1acf ("sched/fair: Check for idle core in wake_affine")
> v2->v3:
> - change setting idle cpumask to every idle entry, otherwise schbench
> has a regression of 99th percentile latency.
> - change clearing idle cpumask to nohz_balancer_kick(), so updating
> idle cpumask is ratelimited in the idle exiting path.
> - set SCHED_IDLE cpu in idle cpumask to allow it as a wakeup target.
>
> v1->v2:
> - idle cpumask is updated in the nohz routines, by initializing idle
> cpumask with sched_domain_span(sd), nohz=off case remains the original
> behavior.
>
> Cc: Mel Gorman <mgor...@suse.de>
> Cc: Vincent Guittot <vincent.guit...@linaro.org>
> Cc: Qais Yousef <qais.you...@arm.com>
> Cc: Valentin Schneider <valentin.schnei...@arm.com>
> Cc: Jiang Biao <benbji...@gmail.com>
> Cc: Tim Chen <tim.c.c...@linux.intel.com>
> Signed-off-by: Aubrey Li <aubrey...@linux.intel.com>
