Re: [PATCH] sched: Optimize housekeeping_cpumask in for_each_cpu_and

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 02, 2021 at 10:03:52AM +0800, Yuan ZhaoXiong wrote:
> On a 128 cores AMD machine, there are 8 cores in nohz_full mode, and
> the others are used for housekeeping. When many housekeeping cpus are
> in idle state, we can observe huge time burn in the loop for searching
> nearest busy housekeeper cpu by ftrace.
> 
>    9)               |              get_nohz_timer_target() {
>    9)               |                housekeeping_test_cpu() {
>    9)   0.390 us    |                  housekeeping_get_mask.part.1();
>    9)   0.561 us    |                }
>    9)   0.090 us    |                __rcu_read_lock();
>    9)   0.090 us    |                housekeeping_cpumask();
>    9)   0.521 us    |                housekeeping_cpumask();
>    9)   0.140 us    |                housekeeping_cpumask();
> 
>    ...
> 
>    9)   0.500 us    |                housekeeping_cpumask();
>    9)               |                housekeeping_any_cpu() {
>    9)   0.090 us    |                  housekeeping_get_mask.part.1();
>    9)   0.100 us    |                  sched_numa_find_closest();
>    9)   0.491 us    |                }
>    9)   0.100 us    |                __rcu_read_unlock();
>    9) + 76.163 us   |              }
> 
> for_each_cpu_and() is a micro function, so in get_nohz_timer_target()
> function the
>         for_each_cpu_and(i, sched_domain_span(sd),
>                 housekeeping_cpumask(HK_FLAG_TIMER))
> equals to below:
>         for (i = -1; i = cpumask_next_and(i, sched_domain_span(sd),
>                 housekeeping_cpumask(HK_FLAG_TIMER)), i < nr_cpu_ids;)
> That will cause that housekeeping_cpumask() will be invoked many times.
> The housekeeping_cpumask() function returns a const value, so it is
> unnecessary to invoke it every time. This patch can minimize the worst
> searching time from ~76us to ~16us in my testing.
> 
> Similarly, the find_new_ilb() function has the same problem.
> 
> Signed-off-by: Yuan ZhaoXiong <yuanzhaoxiong@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>

This is still not a valid SoB chain. Please refer to
Documentation/process/submitting-patches.rst.

The first SoB should match the Author, which if missing is From, the
last SoB should match the Sender which is From. Since there is only one
From but two SoBs this cannot be right.