Re: [External] Re: [PATCH] sched/fair: favor non-idle group in tick preemption

[Hao Jia]

On 2022/11/2 Josh Don wrote:
> > > Some weirdness about this change though, is that if there is a
> > > non-idle current entity, and the two next entities on the cfs_rq are
> > > idle and non-idle respectively, we'll now take longer to preempt the
> > > on-cpu non-idle entity, because the non-idle entity on the cfs_rq is
> > > 'hidden' by the idle 'first' entity. Wakeup preemption is different
> > > because we're always directly comparing the current entity with the
> > > newly woken entity.
> > You are right, this can happen with high probability.
> > This patch just compared the curr with the first entity in
> > the tick, and it seems hard to consider all the other entity
> > in cfs_rq.
> >
> > So, what specific negative effects this situation would cause?
> > For example, the "hidden" non-idle entity's latency will be worse
> > than before?
>
> As Abel points out in his email, it can push out the time it'll take
> to switch to the other non-idle entity. The change might boost some
> benchmarks numbers, but I don't think it is conclusive enough to say
> it is a generically beneficial improvement that should be integrated.
>
> By the way, I'm curious if you modified any of the sched_idle_cpu()
> and related load balancing around idle entities given that you've made
> it so that idle entities can have arbitrary weight (since, as I
> described in my prior email, this can otherwise cause issues there).

If we want to make it easier for non-idle tasks to preempt idle tasks in 
tick, maybe we can consider lowering sysctl_sched_idle_min_granularity. 
Of course this may not ensure that non-idle tasks successfully preempt 
idle tasks every time, but it seems to be more beneficial for non-idle 
tasks.

IMHO, even if it is allowed to increase the weight of non-idle, it seems 
that we can make it easier for non-idle tasks to preempt idle tasks by 
lowering sysctl_sched_idle_min_granularity.

Thanks,
Hao