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

[Abel Wu]

On 7/14/22 3:15 PM, Yicong Yang Wrote:
> On 2022/7/14 14:58, Abel Wu 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@xxxxxxxxxxxxx>
> > > > ---
> > > >    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?
>
> It just makes me think that dereference is unnecessary if this_cpu and target locates in
> the same LLC, since it's already been passed. But since you noticed no difference it may
> have little effect. :)

Hmm.. Not exactly. The sched-domains are cpu private, and this_cpu can
be in another LLC than target.