Re: [RFC PATCH v4 2/3] sched: Avoid placing RT threads on cores handling long softirqs

[Hillf Danton <hdanton@sina.com>]

On 10 Oct 2022 16:42:16 +0100 Qais Yousef <qais.yousef@arm.com>
> On 10/05/22 14:01, Hillf Danton wrote:
> > On 4 Oct 2022 18:13:52 -0700 John Stultz <jstultz@google.com>
> > > On Tue, Oct 4, 2022 at 5:22 PM Hillf Danton <hdanton@sina.com> wrote:
> > > > On 3 Oct 2022 19:29:36 -0700 John Stultz <jstultz@google.com>
> > > > >
> > > > > Why would ksoftirqd preempt the rt task?
> > > > >
> > > > For example the kthread becomes sensitive to latency.
> > > 
> > > Is it the case where
> > > the ksoftirqd thread is configured to run at higher rtprio?
> > > 
> > Yes, you are right.
> 
> I don't see a problem here. If a sys-admin configures their ksoftirqds to be
> a higher priority RT tasks than the audio threads, then they better know what
> they're doing :-)
> 
Thanks for taking a look.

> The issue at hand here is that the softirqs boundedness is hard to control. And
> the scheduling delays ensued are hard to deal with by any sys-admin.
> 
Given "The RT scheduler is for tasks that require strick scheduling
requirements over all else, including performance." [1], I would invite
Steve to shed some more light on the relation between RT scheduler and
performance/network throughputs.

Bonus question, why softirq took no care of RT tasks, given strick
scheduling requirements above?

[1] https://lore.kernel.org/lkml/257E96C2-6ABD-4DD6-9B7F-771DA3D1BAAA@goodmis.org/

> Networking has actually introduced some knobs to help control that - but the
> tricky bit of still being able to deliver high throughput networking while
> keeping the softirq bounded to minimize scheduling delays/latencies. I think
> even for PREEMPT_RT, high performance networking could be impacted to achieve
> the required low latency.
> 
> See this paper which explores this duality:
> 
> 	https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.702.7571&rep=rep1&type=pdf
> 
> 
> With WiFi 6 and 5G mobile networks, phones are actually expected to deliver
> multi-gigabit network throughputs.