Cc Frederic,
On Tue, 18 Jun 2019 at 11:13, Wanpeng Li <[email protected]> wrote:
>
> From: Wanpeng Li <[email protected]>
>
> On a machine, cpu 0 is used for housekeeping, other 39 cpus 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.
>
> Cc: Ingo Molnar <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> kernel/sched/core.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 83bd6bb..db550cf 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -548,11 +548,12 @@ int get_nohz_timer_target(void)
>
> 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;
> }
> --
> 2.7.4
>
