Re: [PATCH] sched/fair: reduce preemption with IDLE tasks runable(Internet mail)

[Vincent Guittot]

On Thu, 20 Aug 2020 at 16:28, Jiang Biao <benbjiang@xxxxxxxxx> wrote:
>
> On Thu, 20 Aug 2020 at 20:46, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> >
> > On Thu, 20 Aug 2020 at 13:28, benbjiang(蒋彪) <benbjiang@xxxxxxxxxxx> wrote:
> > >
> > >
> > >
> > > > On Aug 20, 2020, at 3:35 PM, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, 20 Aug 2020 at 02:13, benbjiang(蒋彪) <benbjiang@xxxxxxxxxxx> wrote:
> > > >>
> > > >>
> > > >>
> > > >>> On Aug 19, 2020, at 10:55 PM, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> > > >>>
> > > >>> On Wed, 19 Aug 2020 at 16:27, benbjiang(蒋彪) <benbjiang@xxxxxxxxxxx> wrote:
> > > >>>>
> > > >>>>
> > > >>>>
> > > >>>>> On Aug 19, 2020, at 7:55 PM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>
> > > >>>>> On 19/08/2020 13:05, Vincent Guittot wrote:
> > > >>>>>> On Wed, 19 Aug 2020 at 12:46, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>
> > > >>>>>>> On 17/08/2020 14:05, benbjiang(蒋彪) wrote:
> > > >>>>>>>>
> > > >>>>>>>>
> > > >>>>>>>>> On Aug 17, 2020, at 4:57 PM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>>>
> > > >>>>>>>>> On 14/08/2020 01:55, benbjiang(蒋彪) wrote:
> > > >>>>>>>>>> Hi,
> > > >>>>>>>>>>
> > > >>>>>>>>>>> On Aug 13, 2020, at 2:39 AM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>>>>>
> > > >>>>>>>>>>> On 12/08/2020 05:19, benbjiang(蒋彪) wrote:
> > > >>>>>>>>>>>> Hi,
> > > >>>>>>>>>>>>
> > > >>>>>>>>>>>>> On Aug 11, 2020, at 11:54 PM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>>>>>>>
> > > >>>>>>>>>>>>> On 11/08/2020 02:41, benbjiang(蒋彪) wrote:
> > > >>>>>>>>>>>>>> Hi,
> > > >>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>> On Aug 10, 2020, at 9:24 PM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>> On 06/08/2020 17:52, benbjiang(蒋彪) wrote:
> > > >>>>>>>>>>>>>>>> Hi,
> > > >>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>> On Aug 6, 2020, at 9:29 PM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>> On 03/08/2020 13:26, benbjiang(蒋彪) wrote:
> > > >>>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>>>> On Aug 3, 2020, at 4:16 PM, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > > >>>>>>>>>>>>>>>>>>>
> > > >>>>>>>>>>>>>>>>>>> On 01/08/2020 04:32, Jiang Biao wrote:
> > > >>>>>>>>>>>>>>>>>>>> From: Jiang Biao <benbjiang@xxxxxxxxxxx>
> > > >>>>>>>
> > > >>>>>>> [...]
> > > >>>>>>>
> > > >>>>>>>>> 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/

>
> Thanks a lot.
> Regards,
> Jiang