Re: [PATCH v3] sched/fair: prefer prev cpu in asymmetric wakeup path

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 29, 2020 at 05:18:24PM +0100, Vincent Guittot wrote:
> During fast wakeup path, scheduler always check whether local or prev cpus
> are good candidates for the task before looking for other cpus in the
> domain. With
>   commit b7a331615d25 ("sched/fair: Add asymmetric CPU capacity wakeup scan")
> the heterogenous system gains a dedicated path but doesn't try to reuse
> prev cpu whenever possible. If the previous cpu is idle and belong to the
> LLC domain, we should check it 1st before looking for another cpu because
> it stays one of the best candidate and this also stabilizes task placement
> on the system.
> 
> This change aligns asymmetric path behavior with symmetric one and reduces
> cases where the task migrates across all cpus of the sd_asym_cpucapacity
> domains at wakeup.
> 
> This change does not impact normal EAS mode but only the overloaded case or
> when EAS is not used.
> 
> - On hikey960 with performance governor (EAS disable)
> 
> ./perf bench sched pipe -T -l 50000
>              mainline           w/ patch
> # migrations   999364                  0
> ops/sec        149313(+/-0.28%)   182587(+/- 0.40) +22%
> 
> - On hikey with performance governor
> 
> ./perf bench sched pipe -T -l 50000
>              mainline           w/ patch
> # migrations        0                  0
> ops/sec         47721(+/-0.76%)    47899(+/- 0.56) +0.4%
> 
> According to test on hikey, the patch doesn't impact symmetric system
> compared to current implementation (only tested on arm64)
> 
> Also read the uclamped value of task's utilization at most twice instead
> instead each time we compare task's utilization with cpu's capacity.
> 
> Fixes: b7a331615d25 ("sched/fair: Add asymmetric CPU capacity wakeup scan")
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---

Thanks!