On 10 Oct 2022 16:42:16 +0100 Qais Yousef <qais.yousef@xxxxxxx> > On 10/05/22 14:01, Hillf Danton wrote: > > On 4 Oct 2022 18:13:52 -0700 John Stultz <jstultz@xxxxxxxxxx> > > > On Tue, Oct 4, 2022 at 5:22 PM Hillf Danton <hdanton@xxxxxxxx> wrote: > > > > On 3 Oct 2022 19:29:36 -0700 John Stultz <jstultz@xxxxxxxxxx> > > > > > > > > > > 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@xxxxxxxxxxx/ > 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.
