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

[Vincent Guittot]

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

> Or introduce a switch(or flag) to control it, and make it available for cases like us.
>
> Thanks a lot.
> Regards,
> Jiang
>
> >>
> >>>
> >>>> OTOH, cgroups(shares) could introduce extra complexity. :)
> >>>>
> >>>> I wonder if there’s any possibility to make SCHED_IDLEs’ priorities absolutely
> >>>> lower than SCHED_NORMAL(OTHER), which means no weights/shares
> >>>> for them, and they run only when no other task’s runnable.
> >>>> I guess there may be priority inversion issue if we do that. But maybe we
> >>>
> >>> Exactly, that's why we must ensure a minimum running time for sched_idle task
> >>
> >> Still for VM case, different VMs have been much isolated from each other,
> >> priority inversion issue could be very rare, we’re trying to make offline tasks
> >> absoultly harmless to online tasks. :)
> >>
> >> Thanks a lot for your time.
> >> Regards,
> >> Jiang
> >>
> >>>
> >>>> could avoid it by load-balance more aggressively, or it(priority inversion)
> >>>> could be ignored in some special case.
>