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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Aubrey Li <aubrey.li@linux.intel.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        tim.c.chen@linux.intel.com, linux-kernel@vger.kernel.org,
        Aubrey Li <aubrey.li@intel.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Jiang Biao <benbjiang@gmail.com>
Subject: Re: [RFC PATCH v3] sched/fair: select idle cpu from idle cpumask for task wakeup
Message-ID: <jhj1rh6yygz.mognet@arm.com>
In-reply-to: <20201021150335.1103231-1-aubrey.li@linux.intel.com>
Date:   Fri, 06 Nov 2020 21:20:05 +0000
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On 21/10/20 16:03, Aubrey Li wrote:
> From: Aubrey Li <aubrey.li@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 <mgorman@suse.de>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Qais Yousef <qais.yousef@arm.com>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Jiang Biao <benbjiang@gmail.com>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>