On Thu, 20 Aug 2020 at 22:36, Vincent Guittot <vincent.guit...@linaro.org> wrote: > > On Thu, 20 Aug 2020 at 16:28, Jiang Biao <benbji...@gmail.com> wrote: > > > > On Thu, 20 Aug 2020 at 20:46, Vincent Guittot > > <vincent.guit...@linaro.org> wrote: > > > > > > On Thu, 20 Aug 2020 at 13:28, benbjiang(蒋彪) <benbji...@tencent.com> wrote: > > > > > > > > > > > > > > > > > On Aug 20, 2020, at 3:35 PM, Vincent Guittot > > > > > <vincent.guit...@linaro.org> wrote: > > > > > > > > > > On Thu, 20 Aug 2020 at 02:13, benbjiang(蒋彪) <benbji...@tencent.com> > > > > > wrote: > > > > >> > > > > >> > > > > >> > > > > >>> On Aug 19, 2020, at 10:55 PM, Vincent Guittot > > > > >>> <vincent.guit...@linaro.org> wrote: > > > > >>> > > > > >>> On Wed, 19 Aug 2020 at 16:27, benbjiang(蒋彪) <benbji...@tencent.com> > > > > >>> wrote: > > > > >>>> > > > > >>>> > > > > >>>> > > > > >>>>> On Aug 19, 2020, at 7:55 PM, Dietmar Eggemann > > > > >>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>> > > > > >>>>> On 19/08/2020 13:05, Vincent Guittot wrote: > > > > >>>>>> On Wed, 19 Aug 2020 at 12:46, Dietmar Eggemann > > > > >>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>> > > > > >>>>>>> On 17/08/2020 14:05, benbjiang(蒋彪) wrote: > > > > >>>>>>>> > > > > >>>>>>>> > > > > >>>>>>>>> On Aug 17, 2020, at 4:57 PM, Dietmar Eggemann > > > > >>>>>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>>>> > > > > >>>>>>>>> On 14/08/2020 01:55, benbjiang(蒋彪) wrote: > > > > >>>>>>>>>> Hi, > > > > >>>>>>>>>> > > > > >>>>>>>>>>> On Aug 13, 2020, at 2:39 AM, Dietmar Eggemann > > > > >>>>>>>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>>>>>> > > > > >>>>>>>>>>> On 12/08/2020 05:19, benbjiang(蒋彪) wrote: > > > > >>>>>>>>>>>> Hi, > > > > >>>>>>>>>>>> > > > > >>>>>>>>>>>>> On Aug 11, 2020, at 11:54 PM, Dietmar Eggemann > > > > >>>>>>>>>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>>>>>>>> > > > > >>>>>>>>>>>>> On 11/08/2020 02:41, benbjiang(蒋彪) wrote: > > > > >>>>>>>>>>>>>> Hi, > > > > >>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>> On Aug 10, 2020, at 9:24 PM, Dietmar Eggemann > > > > >>>>>>>>>>>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>> On 06/08/2020 17:52, benbjiang(蒋彪) wrote: > > > > >>>>>>>>>>>>>>>> Hi, > > > > >>>>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>>>> On Aug 6, 2020, at 9:29 PM, Dietmar Eggemann > > > > >>>>>>>>>>>>>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>>>> On 03/08/2020 13:26, benbjiang(蒋彪) wrote: > > > > >>>>>>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>>>>>> On Aug 3, 2020, at 4:16 PM, Dietmar Eggemann > > > > >>>>>>>>>>>>>>>>>>> <dietmar.eggem...@arm.com> wrote: > > > > >>>>>>>>>>>>>>>>>>> > > > > >>>>>>>>>>>>>>>>>>> On 01/08/2020 04:32, Jiang Biao wrote: > > > > >>>>>>>>>>>>>>>>>>>> From: Jiang Biao <benbji...@tencent.com> > > > > >>>>>>> > > > > >>>>>>> [...] > > > > >>>>>>> > > > > >>>>>>>>> Are you sure about this? > > > > >>>>>>>> Yes. :) > > > > >>>>>>>>> > > > > >>>>>>>>> The math is telling me for the: > > > > >>>>>>>>> > > > > >>>>>>>>> idle task: (3 / (1024 + 1024 + 3))^(-1) * 4ms = 2735ms > > > > >>>>>>>>> > > > > >>>>>>>>> normal task: (1024 / (1024 + 1024 + 3))^(-1) * 4ms = 8ms > > > > >>>>>>>>> > > > > >>>>>>>>> (4ms - 250 Hz) > > > > >>>>>>>> My tick is 1ms - 1000HZ, which seems reasonable for 600ms? :) > > > > >>>>>>> > > > > >>>>>>> OK, I see. > > > > >>>>>>> > > > > >>>>>>> But here the different sched slices (check_preempt_tick()-> > > > > >>>>>>> sched_slice()) between normal tasks and the idle task play a > > > > >>>>>>> role to. > > > > >>>>>>> > > > > >>>>>>> Normal tasks get ~3ms whereas the idle task gets <0.01ms. > > > > >>>>>> > > > > >>>>>> In fact that depends on the number of CPUs on the system > > > > >>>>>> :sysctl_sched_latency = 6ms * (1 + ilog(ncpus)) . On a 8 cores > > > > >>>>>> system, > > > > >>>>>> normal task will run around 12ms in one shoot and the idle task > > > > >>>>>> still > > > > >>>>>> one tick period > > > > >>>>> > > > > >>>>> True. This is on a single CPU. > > > > >>>> Agree. :) > > > > >>>> > > > > >>>>> > > > > >>>>>> Also, you can increase even more the period between 2 runs of > > > > >>>>>> idle > > > > >>>>>> task by using cgroups and min shares value : 2 > > > > >>>>> > > > > >>>>> Ah yes, maybe this is what Jiang wants to do then? If his runtime > > > > >>>>> does > > > > >>>>> not have other requirements preventing this. > > > > >>>> That could work for increasing the period between 2 runs. But > > > > >>>> could not > > > > >>>> reduce the single runtime of idle task I guess, which means normal > > > > >>>> task > > > > >>>> could have 1-tick schedule latency because of idle task. > > > > >>> > > > > >>> Yes. An idle task will preempt an always running task during 1 tick > > > > >>> every 680ms. But also you should keep in mind that a waking normal > > > > >>> task will preempt the idle task immediately which means that it will > > > > >>> not add scheduling latency to a normal task but "steal" 0.14% of > > > > >>> normal task throughput (1/680) at most > > > > >> That’s true. But in the VM case, when VM are busy(MWAIT passthrough > > > > >> or running cpu eating works), the 1-tick scheduling latency could be > > > > >> detected by cyclictest running in the VM. > > > > >> > > > > >> OTOH, we compensate vruntime in place_entity() to boot waking > > > > >> without distinguish SCHED_IDLE task, do you think it’s necessary to > > > > >> do that? like > > > > >> > > > > >> --- a/kernel/sched/fair.c > > > > >> +++ b/kernel/sched/fair.c > > > > >> @@ -4115,7 +4115,7 @@ place_entity(struct cfs_rq *cfs_rq, struct > > > > >> sched_entity *se, int initial) > > > > >> vruntime += sched_vslice(cfs_rq, se); > > > > >> > > > > >> /* sleeps up to a single latency don't count. */ > > > > >> - if (!initial) { > > > > >> + if (!initial && likely(!task_has_idle_policy(task_of(se)))) { > > > > >> unsigned long thresh = sysctl_sched_latency; > > > > > > > > > > Yeah, this is a good improvement. > > > > Thanks, I’ll send a patch for that. :) > > > > > > > > > Does it solve your problem ? > > > > > > > > > Not exactly. :) I wonder if we can make SCHED_IDLE more pure(harmless)? > > > > > > We can't prevent it from running time to time. Proxy execution feature > > > could be a step for considering to relax this constraint > > > > > Could you please help to explain more about the *Proxy execution feature*? > > I'm not sure I got the right point. > > https://lwn.net/Articles/820575/ > https://lwn.net/Articles/793502/ Good to hear about that. I guess there would still be a long way to go. :)
Thanks again for your kindly patience. Regard, Jiang