Messages in this thread | | | Subject | Re: [RFC PATCH v2] sched/fair: select idle cpu from idle cpumask in sched domain | From | "Li, Aubrey" <> | Date | Sun, 27 Sep 2020 13:56:02 +0800 |
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On 2020/9/26 0:45, Vincent Guittot wrote: > 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 Vincent, let me try to refine this patch.
-Aubrey
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