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

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Date:   Mon, 10 Oct 2022 16:42:16 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     John Stultz <jstultz@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Connor O'Brien <connoro@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH v4 2/3] sched: Avoid placing RT threads on cores
 handling long softirqs

On 10/05/22 14:01, Hillf Danton wrote:
> On 4 Oct 2022 18:13:52 -0700 John Stultz <jstultz@...gle.com>
> > On Tue, Oct 4, 2022 at 5:22 PM Hillf Danton <hdanton@...a.com> wrote:
> > > On 3 Oct 2022 19:29:36 -0700 John Stultz <jstultz@...gle.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