Re: [RFC PATCH v2] sched/fair: select idle cpu from idle cpumask in sched domain

[Vincent Guittot]

Le vendredi 25 sept. 2020 à 17:21:46 (+0800), Li, Aubrey a écrit :
> Hi Vicent,
> 
> On 2020/9/24 21:09, Vincent Guittot wrote:
> >>>>
> >>>> Would you mind share uperf(netperf load) result on your side? That's the
> >>>> workload I have seen the most benefit this patch contributed under heavy
> >>>> load level.
> >>>
> >>> with uperf, i've got the same kind of result as sched pipe
> >>> tip/sched/core: Throughput 24.83Mb/s (+/- 0.09%)
> >>> with this patch:  Throughput 19.02Mb/s (+/- 0.71%) which is a 23%
> >>> regression as for sched pipe
> >>>
> >> In case this is caused by the logic error in this patch(sorry again), did
> >> you see any improvement in patch V2? Though it does not helps for nohz=off
> >> case, just want to know if it helps or does not help at all on arm platform.
> > 
> > With the v2 which rate limit the update of the cpumask (but doesn't
> > support sched_idle stask),  I don't see any performance impact:
> 
> I agree we should go the way with cpumask update rate limited.
> 
> And I think no performance impact for sched-pipe is expected, as this workload
> has only 2 threads and the platform has 8 cores, so mostly previous cpu is
> returned, and even if select_idle_sibling is called, select_idle_core is hit
> and rarely call select_idle_cpu.

my platform is not smt so select_idle_core is nop. Nevertheless select_idle_cpu
is almost never called because prev is idle and selected before calling it in
our case

> 
> But I'm more curious why there is 23% performance penalty? So for this patch, if
> you revert this change but keep cpumask updated, is 23% penalty still there?
> 
> -       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +       cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);

I was about to say that reverting this line should not change anything because
we never reach this point but it does in fact. And after looking at a trace,
I can see that the 2 threads of perf bench sched pipe are on the same CPU and
that the sds_idle_cpus(sd->shared) is always empty. In fact, the rq->curr is
not yet idle and still point to the cfs task when you call update_idle_cpumask().
This means that once cleared, the bit will never be set
You can remove the test in update_idle_cpumask() which is called either when
entering idle or when there is only sched_idle tasks that are runnable.

@@ -6044,8 +6044,7 @@ void update_idle_cpumask(struct rq *rq)
        sd = rcu_dereference(per_cpu(sd_llc, cpu));
        if (!sd || !sd->shared)
                goto unlock;
-       if (!available_idle_cpu(cpu) || !sched_idle_cpu(cpu))
-               goto unlock;
+
        cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
 unlock:
        rcu_read_unlock();

With this fix, the performance decrease is only 2%

> 
> I just wonder if it's caused by the atomic ops as you have two cache domains with
> sd_llc(?). Do you have a x86 machine to make a comparison? It's hard for me to find
> an ARM machine but I'll try.
> 
> Also, for uperf(task thread num = cpu num) workload, how is it on patch v2? no any
> performance impact?

with v2 :  Throughput 24.97Mb/s (+/- 0.07%) so there is no perf regression

>
> 
> Thanks,
> -Aubrey