On Fri, Jun 28, 2019 at 08:43:12AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpen...@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 <mi...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Frederic Weisbecker <frede...@kernel.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Signed-off-by: Wanpeng Li <wanpen...@tencent.com>
Reviewed-by: Frederic Weisbecker <frede...@kernel.org>
Thanks!
