linux-kernel - Re: [PATCH] cpufreq: Change default transition delay to 2ms

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Message-ID: <326b568d-d460-4a69-9336-28da328ffdcf@arm.com>
Date: Mon, 5 Feb 2024 09:17:45 +0000
From: Christian Loehle <christian.loehle@....com>
To: Qais Yousef <qyousef@...alina.io>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
 Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH] cpufreq: Change default transition delay to 2ms

On 05/02/2024 02:25, Qais Yousef wrote:
> 10ms is too high for today's hardware, even low end ones. This default
> end up being used a lot on Arm machines at least. Pine64, mac mini and
> pixel 6 all end up with 10ms rate_limit_us when using schedutil, and
> it's too high for all of them.
> 
> Change the default to 2ms which should be 'pessimistic' enough for worst
> case scenario, but not too high for platforms with fast DVFS hardware.
> 
> Signed-off-by: Qais Yousef <qyousef@...alina.io>
> ---
>  drivers/cpufreq/cpufreq.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 44db4f59c4cc..8207f7294cb6 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -582,11 +582,11 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
>  		 * for platforms where transition_latency is in milliseconds, it
>  		 * ends up giving unrealistic values.
>  		 *
> -		 * Cap the default transition delay to 10 ms, which seems to be
> +		 * Cap the default transition delay to 2 ms, which seems to be
>  		 * a reasonable amount of time after which we should reevaluate
>  		 * the frequency.
>  		 */
> -		return min(latency * LATENCY_MULTIPLIER, (unsigned int)10000);
> +		return min(latency * LATENCY_MULTIPLIER, (unsigned int)(2*MSEC_PER_SEC));
>  	}
>  
>  	return LATENCY_MULTIPLIER;

Hi Qais,
as previously mentioned I'm working on improving iowait boost and while I'm not against
this patch per se it does make iowait boosting more aggressive. ((Doubling limited by rate_limit_us)
Since the boost is often applied when not useful (for Android e.g. periodic f2fs writebacks),
this might have some side effects. Please give me a couple of days for verifying any impact,
or did you do that already?

Kind Regards,
Christian