Re: [PATCH v2] sched/nohz: Optimize get_nohz_timer_target()

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Jun 28, 2019 at 08:43:12AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> On a machine, cpu 0 is used for housekeeping, the other 39 cpus in the 
> same socket are in nohz_full mode. We can observe huge time burn in the 
> loop for seaching nearest busy housekeeper cpu by ftrace.
> 
>   2)               |       get_nohz_timer_target() {
>   2)   0.240 us    |         housekeeping_test_cpu();
>   2)   0.458 us    |         housekeeping_test_cpu();
> 
>   ...
> 
>   2)   0.292 us    |         housekeeping_test_cpu();
>   2)   0.240 us    |         housekeeping_test_cpu();
>   2)   0.227 us    |         housekeeping_any_cpu();
>   2) + 43.460 us   |       }
>   
> This patch optimizes the searching logic by finding a nearest housekeeper
> cpu in the housekeeping cpumask, it can minimize the worst searching time 
> from ~44us to < 10us in my testing. In addition, the last iterated busy 
> housekeeper can become a random candidate while current CPU is a better 
> fallback if it is a housekeeper.
> 
> Cc: Ingo Molnar <mingo@redhat.com> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

Thanks!