linux-kernel - Re: [PATCH v2] sched/nohz: Optimize get_nohz_timer

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Date:   Fri, 28 Jun 2019 03:10:13 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched/nohz: Optimize get_nohz_timer_target()

On Fri, Jun 28, 2019 at 08:43:12AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.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@...hat.com> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>

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

Thanks!