linux-kernel - Re: [PATCH v3 1/4] cpufreq: Make drivers using CPUFREQ

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Message-ID: <20251001103815.a57cc3wyasyhmrmt@airbuntu>
Date: Wed, 1 Oct 2025 11:38:15 +0100
From: Qais Yousef <qyousef@...alina.io>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Linux PM <linux-pm@...r.kernel.org>, Shawn Guo <shawnguo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Pierre Gondois <pierre.gondois@....com>,
	Mario Limonciello <mario.limonciello@....com>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	Jie Zhan <zhanjie9@...ilicon.com>, rust-for-linux@...r.kernel.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>
Subject: Re: [PATCH v3 1/4] cpufreq: Make drivers using CPUFREQ_ETERNAL
 specify transition latency

On 09/26/25 12:12, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Commit a755d0e2d41b ("cpufreq: Honour transition_latency over
> transition_delay_us") caused platforms where cpuinfo.transition_latency
> is CPUFREQ_ETERNAL to get a very large transition latency whereas
> previously it had been capped at 10 ms (and later at 2 ms).
> 
> This led to a user-observable regression between 6.6 and 6.12 as
> described by Shawn:
> 
> "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 the default transition delay was dropped [...].
> 
>  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."
> 
> Evidently, the expectation of the drivers using CPUFREQ_ETERNAL as
> cpuinfo.transition_latency was that it would be capped by the core,
> but they may as well return a default transition latency value instead
> of CPUFREQ_ETERNAL and the core need not do anything with it.
> 
> Accordingly, introduce CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS and make
> all of the drivers in question use it instead of CPUFREQ_ETERNAL.  Also
> update the related Rust binding.
> 
> Fixes: a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us")
> Closes: https://lore.kernel.org/linux-pm/20250922125929.453444-1-shawnguo2@yeah.net/
> Reported-by: Shawn Guo <shawnguo@...nel.org>
> Reviewed-by: Mario Limonciello (AMD) <superm1@...nel.org>
> Reviewed-by: Jie Zhan <zhanjie9@...ilicon.com>
> Cc: 6.6+ <stable@...r.kernel.org> # 6.6+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

The whole series LGTM. I think this is clearer now without the CPUFREQ_ETERNAL.

Reviewed-by: Qais Yousef <qyousef@...alina.io>


Cheers

--
Qais Yousef