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From:   John Ogness <john.ogness@linutronix.de>
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To:     Gautam Thaker <ghthaker@gmail.com>
Cc:     linux-rt-users@vger.kernel.org
Subject: Re: 5.15.28-rt35 #2 SMP PREEMPT_RT: Should scheduling latency be as
 large as 800 usec?
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Date:   Fri, 25 Mar 2022 09:24:53 +0106
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On 2022-03-24, Gautam Thaker <ghthaker@gmail.com> wrote:
> On Thu, Mar 24, 2022 at 1:55 AM John Ogness <john.ogness@linutronix.de> wrote:
> I tried both
> cpufreq-set -g performance
> and
> cpufreq-set -g schedutil
>
> And also set the lower and upper freq to 3.2 GHz. Overall this has not
> made much of a  difference, I still see ~800 usec latency reports out
> of 'hwlatdetect'. The best run was:
>
> ...
> ... [Same for CPUs 0-30 ...]
> analyzing CPU 31:
>   driver: intel_cpufreq
>   CPUs which run at the same hardware frequency: 31
>   CPUs which need to have their frequency coordinated by software: 31
>   maximum transition latency: 20.0 us.
>   hardware limits: 1.20 GHz - 3.20 GHz
>   available cpufreq governors: conservative, ondemand, userspace,
> powersave, performance, schedutil
>   current policy: frequency should be within 3.20 GHz and 3.20 GHz.
>                   The governor "schedutil" may decide which speed to use
>                   within this range.
>   current CPU frequency is 1.20 GHz.

With the performance governor you will see that "current CPU frequency"
is _always_ the max. Without that, I cannot imagine eliminating >100us
latencies.

John Ogness