linux-kernel - Re: [PATCH] cpufreq: cap the default transition delay at 10 ms

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Message-ID: <CAJZ5v0gL5s99h0eq1U4ngaUfPq_AcfgPruSD096JtBWVMjSZwQ@mail.gmail.com>
Date: Fri, 12 Sep 2025 12:41:14 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Shawn Guo <shawnguo2@...h.net>
Cc: "Rafael J . Wysocki" <rafael@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>, 
	Qais Yousef <qyousef@...alina.io>, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Shawn Guo <shawnguo@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] cpufreq: cap the default transition delay at 10 ms

On Wed, Sep 10, 2025 at 8:53 AM Shawn Guo <shawnguo2@...h.net> wrote:
>
> From: Shawn Guo <shawnguo@...nel.org>
>
> A regression is seen with 6.6 -> 6.12 kernel upgrade on platforms where
> cpufreq-dt driver sets cpuinfo.transition_latency as CPUFREQ_ETERNAL (-1),
> due to that platform's DT doesn't provide the optional property
> 'clock-latency-ns'.  The dbs sampling_rate was 10000 us on 6.6 and
> suddently becomes 6442450 us (4294967295 / 1000 * 1.5) on 6.12 for these
> platforms, because that the 10 ms cap for transition_delay_us was
> accidentally dropped by the commits below.

IIRC, this was not accidental.

Why do you want to address the issue in the cpufreq core instead of
doing that in the cpufreq-dt driver?

CPUFREQ_ETERNAL doesn't appear to be a reasonable default for
cpuinfo.transition_latency.  Maybe just change the default there to 10
ms?

>   commit 37c6dccd6837 ("cpufreq: Remove LATENCY_MULTIPLIER")
>   commit a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us")
>   commit e13aa799c2a6 ("cpufreq: Change default transition delay to 2ms")
>
> It slows down dbs governor's reacting to CPU loading change
> dramatically.  Also, as transition_delay_us is used by schedutil governor
> as rate_limit_us, it shows a negative impact on device idle power
> consumption, because the device gets slightly less time in the lowest OPP.
>
> Fix the regressions by adding the 10 ms cap on transition delay back.
>
> Cc: stable@...r.kernel.org
> Fixes: 37c6dccd6837 ("cpufreq: Remove LATENCY_MULTIPLIER")
> Signed-off-by: Shawn Guo <shawnguo@...nel.org>
> ---
>  drivers/cpufreq/cpufreq.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index fc7eace8b65b..36e0c85cb4e0 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -551,8 +551,13 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
>
>         latency = policy->cpuinfo.transition_latency / NSEC_PER_USEC;
>         if (latency)
> -               /* Give a 50% breathing room between updates */
> -               return latency + (latency >> 1);
> +               /*
> +                * Give a 50% breathing room between updates.
> +                * And cap the transition delay to 10 ms for platforms
> +                * where the latency is too high to be reasonable for
> +                * reevaluating frequency.
> +                */
> +               return min(latency + (latency >> 1), 10 * MSEC_PER_SEC);
>
>         return USEC_PER_MSEC;
>  }
> --
> 2.43.0
>