Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core

[Chen Yu <yu.chen.surf@gmail.com>]

On Thu, Jul 14, 2022 at 3:11 PM Abel Wu <wuyun.abel@bytedance.com> wrote:
>
>
> On 7/14/22 2:19 PM, Yicong Yang Wrote:
> > On 2022/7/12 16:20, Abel Wu wrote:
> >> When SIS_UTIL is enabled, SIS domain scan will be skipped if
> >> the LLC is overloaded. Since the overloaded status is checked
> >> in the load balancing at LLC level, the interval is llc_size
> >> miliseconds. The duration might be long enough to affect the
> >> overall system throughput if idle cores are out of reach in
> >> SIS domain scan.
> >>
> >> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> >> ---
> >>   kernel/sched/fair.c | 15 +++++++++------
> >>   1 file changed, 9 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index a78d2e3b9d49..cd758b3616bd 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -6392,16 +6392,19 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>      struct sched_domain *this_sd;
> >>      u64 time = 0;
> >>
> >> -    this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> >> -    if (!this_sd)
> >> -            return -1;
> >> -
> >>      cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >>
> >> -    if (sched_feat(SIS_PROP) && !has_idle_core) {
> >> +    if (has_idle_core)
> >> +            goto scan;
> >> +
> >> +    if (sched_feat(SIS_PROP)) {
> >>              u64 avg_cost, avg_idle, span_avg;
> >>              unsigned long now = jiffies;
> >>
> >> +            this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
> >> +            if (!this_sd)
> >> +                    return -1;
> >> +
> >
> > I don't follow the change here. True that this_sd is used only in SIS_PROP, but it seems irrelevant with your
> > commit. Does the position of this make any performance difference?
>
> No, this change doesn't make much difference to performance. Are
> you suggesting that I should make this a separate patch?
>
I took a look at this patch again before I start a OLTP test. I
thought the position change of
dereference sd_llc might not be closely connected with current change
as Yicong mentioned.
Besides, after moving the dereference inside SIS_PROP, we might do
cpumask_and() no matter whether
sd_llc is valid or not, which might be of extra cost?

thanks,
Chenyu

> Thanks,
> Abel
>
> >
> > Thanks.
> >
> >>              /*
> >>               * If we're busy, the assumption that the last idle period
> >>               * predicts the future is flawed; age away the remaining
> >> @@ -6436,7 +6439,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>                              return -1;
> >>              }
> >>      }
> >> -
> >> +scan:
> >>      for_each_cpu_wrap(cpu, cpus, target + 1) {
> >>              if (has_idle_core) {
> >>                      i = select_idle_core(p, cpu, cpus, &idle_cpu);
> >>



-- 
Thanks,
Chenyu