Re: [PATCH 1/1] sched/rt: avoid contend with CFS task

Vincent Guittot
2019-09-19T14:37:27Z

On Thu, 19 Sep 2019 at 16:32, Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> On Thu, 19 Sep 2019 at 16:23, Qais Yousef <qais.yousef@arm.com> wrote:
> >
> > On 09/19/19 14:27, Vincent Guittot wrote:
> > > > > > But for requirement of performance, I think it is better to differentiate between idle CPU and CPU has CFS task.
> > > > > >
> > > > > > For example, we use rt-app to evaluate runnable time on non-patched environment.
> > > > > > There are (NR_CPUS-1) heavy CFS tasks and 1 RT Task. When a CFS task is running, the RT task wakes up and choose the same CPU.
> > > > > > The CFS task will be preempted and keep runnable until it is migrated to another cpu by load balance.
> > > > > > But load balance is not triggered immediately, it will be triggered until timer tick hits with some condition satisfied(ex. rq->next_balance).
> > > > >
> > > > > Yes you will have to wait for the next tick that will trigger an idle
> > > > > load balance because you have an idle cpu and 2 runnable tack (1 RT +
> > > > > 1CFS) on the same CPU. But you should not wait for more than 1 tick
> > > > >
> > > > > The current load_balance doesn't handle correctly the situation of 1
> > > > > CFS and 1 RT task on same CPU while 1 CPU is idle. There is a rework
> > > > > of the load_balance that is under review on the mailing list that
> > > > > fixes this problem and your CFS task should migrate to the idle CPU
> > > > > faster than now
> > > > >
> > > >
> > > > Period load balance should be triggered when current jiffies is behind
> > > > rq->next_balance, but rq->next_balance is not often exactly the same
> > > > with next tick.
> > > > If cpu_busy, interval = sd->balance_interval * sd->busy_factor, and
> > >
> > > But if there is an idle CPU on the system, the next idle load balance
> > > should apply shortly because the busy_factor is not used for this CPU
> > > which is not busy.
> > > In this case, the next_balance interval is sd_weight which is probably
> > > 4ms at cluster level and 8ms at system level in your case. This means
> > > between 1 and 2 ticks
> >
> > But if the CFS task we're preempting was latency sensitive - this 1 or 2 tick
> > is too late of a recovery.
> >
> > So while it's good we recover, but a preventative approach would be useful too.
> > Just saying :-) I'm still not sure if this is the best longer term approach.
>
> like using a rt task ?

I mean, RT task should select a sub optimal CPU because of CFS
If you want to favor CFS compared to RT it's probably because your
task should be RT too

>
> >
> > --
> > Qais Yousef