Messages in this thread | | | Date | Mon, 10 Oct 2022 16:42:16 +0100 | From | Qais Yousef <> | Subject | Re: [RFC PATCH v4 2/3] sched: Avoid placing RT threads on cores handling long softirqs |
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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 :-)
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.
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.
Cheers
-- Qais Yousef
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