lkml.org 
[lkml]   [2020]   [Jan]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH RESEND v2] sched/nohz: Optimize get_nohz_timer_target()
ping Thomas,
On Mon, 13 Jan 2020 at 08:50, Wanpeng Li <kernellwp@gmail.com> 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.
>
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v1 -> v2:
> * current CPU is a better fallback if it is a housekeeper
>
> kernel/sched/core.c | 19 ++++++++++++-------
> 1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 102dfcf..04a0f6a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -539,27 +539,32 @@ void resched_cpu(int cpu)
> */
> int get_nohz_timer_target(void)
> {
> - int i, cpu = smp_processor_id();
> + int i, cpu = smp_processor_id(), default_cpu = -1;
> struct sched_domain *sd;
>
> - if (!idle_cpu(cpu) && housekeeping_cpu(cpu, HK_FLAG_TIMER))
> - return cpu;
> + if (housekeeping_cpu(cpu, HK_FLAG_TIMER)) {
> + if (!idle_cpu(cpu))
> + return cpu;
> + default_cpu = cpu;
> + }
>
> rcu_read_lock();
> for_each_domain(cpu, sd) {
> - for_each_cpu(i, sched_domain_span(sd)) {
> + for_each_cpu_and(i, sched_domain_span(sd),
> + housekeeping_cpumask(HK_FLAG_TIMER)) {
> if (cpu == i)
> continue;
>
> - if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
> + if (!idle_cpu(i)) {
> cpu = i;
> goto unlock;
> }
> }
> }
>
> - if (!housekeeping_cpu(cpu, HK_FLAG_TIMER))
> - cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
> + if (default_cpu == -1)
> + default_cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
> + cpu = default_cpu;
> unlock:
> rcu_read_unlock();
> return cpu;
> --
> 1.8.3.1
>

\
 
 \ /
  Last update: 2020-01-20 07:15    [W:0.090 / U:8.728 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site