linux-kernel - Re: [RFC PATCH v4 3/3] softirq: defer softirq processing to ksoftirqd if CPU is busy with RT

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Date:   Mon, 17 Oct 2022 17:04:09 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Dias <joaodias@...gle.com>,
        "Connor O'Brien" <connoro@...gle.com>,
        Rick Yiu <rickyiu@...gle.com>, John Kacur <jkacur@...hat.com>,
        Chris Redpath <chris.redpath@....com>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        "J . Avila" <elavila@...gle.com>
Subject: Re: [RFC PATCH v4 3/3] softirq: defer softirq processing to ksoftirqd
 if CPU is busy with RT

On Mon, Oct 17, 2022 at 7:45 AM Qais Yousef <qais.yousef@....com> wrote:
> This time I paid attention to the average as the best case number for vanilla
> kernel is better:
>
>                    |       vanilla      | with softirq patches v4  |
> -------------------|--------------------|--------------------------|
>                    |  #1  |  #2  |  #3  |   #1   |   #2   |   #3   |
> -------------------|------|------|------|--------|--------|--------|
> t0 avg delay (us)  |31.59 |22.94 |26.50 | 31.81  | 33.57  | 34.90  |
> t1 avg delay (us)  |16.85 |16.32 |37.16 | 29.05  | 30.51  | 31.65  |
> t2 avg delay (us)  |25.34 |32.12 |17.40 | 26.76  | 28.28  | 28.56  |
>
> It shows that we largely hover around 30us with the patches compared to 16-26us
> being more prevalent for vanilla kernels.
>
> I am not sure I can draw a concrete conclusion from these numbers. It seems
> I need to run longer than 4 hours to hit the worst case scenario every run on
> the vanilla kernel. There's an indication that the worst case scenario is
> harder to hit, and it looks there's a hit on the average delay.

Thanks so much for running these tests and capturing these detailed numbers!

I'll have to look further into the average case going up here.

> I'm losing access to this system from today. I think I'll wait for more
> feedback on this RFC; and do another round of testing for longer periods of
> time once there's clearer sense this is indeed the direction we'll be going
> for.

Do you mind sending me the script you used to run the test, and I'll
try to reproduce on some x86 hardware locally?

thanks
-john