Re: [PATCH] cpufreq: sched/schedutil: Remove LATENCY_MULTIPLIER

[Pierre Gondois]

Hello,
Just to provide some background, there was a previous thread on the same topic at:
  https://lore.kernel.org/lkml/20240205022500.2232124-1-qyousef@xxxxxxxxxxx/

Regards,
Pierre

On 7/28/24 21:26, Qais Yousef wrote:
> The current LATENCY_MULTIPLIER which has been around for nearly 20 years
> causes rate_limit_us to be always in ms range.
>
> On M1 mac mini I get 50 and 56us transition latency, but due to the 1000
> multiplier we end up setting rate_limit_us to 50 and 56ms, which gets
> capped into 2ms and was 10ms before e13aa799c2a6 ("cpufreq: Change
> default transition delay to 2ms")
>
> On Intel I5 system transition latency is 20us but due to the multiplier
> we end up with 20ms that again is capped to 2ms.
>
> Given how good modern hardware and how modern workloads require systems
> to be more responsive to cater for sudden changes in workload (tasks
> sleeping/wakeup/migrating, uclamp causing a sudden boost or cap) and
> that 2ms is quarter of the time of 120Hz refresh rate system, drop the
> old logic in favour of providing 50% headroom.
>
> 	rate_limit_us = 1.5 * latency.
>
> I considered not adding any headroom which could mean that we can end up
> with infinite back-to-back requests.
>
> I also considered providing a constant headroom (e.g: 100us) assuming
> that any h/w or f/w dealing with the request shouldn't require a large
> headroom when transition_latency is actually high.
>
> But for both cases I wasn't sure if h/w or f/w can end up being
> overwhelmed dealing with the freq requests in a potentially busy system.
> So I opted for providing 50% breathing room.
>
> This is expected to impact schedutil only as the other user,
> dbs_governor, takes the max(2*tick, transition_delay_us) and the former
> was at least 2ms on 1ms TICK, which is equivalent to the max_delay_us
> before applying this patch. For systems with TICK of 4ms, this value
> would have almost always ended up with 8ms sampling rate.
>
> For systems that report 0 transition latency, we still default to
> returning 1ms as transition delay.
>
> This helps in eliminating a source of latency for applying requests as
> mentioned in [1]. For example if we have a 1ms tick, most systems will
> miss sending an update at tick when updating the util_avg for a task/CPU
> (rate_limit_us will be 2ms for most systems).
>
> [1] https://lore.kernel.org/lkml/20240724212255.mfr2ybiv2j2uqek7@airbuntu/
>
> Signed-off-by: Qais Yousef <qyousef@xxxxxxxxxxx>
> ---
>   drivers/cpufreq/cpufreq.c | 27 ++++-----------------------
>   include/linux/cpufreq.h   |  6 ------
>   2 files changed, 4 insertions(+), 29 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 04fc786dd2c0..f98c9438760c 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -575,30 +575,11 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
>   		return policy->transition_delay_us;
>   
>   	latency = policy->cpuinfo.transition_latency / NSEC_PER_USEC;
> -	if (latency) {
> -		unsigned int max_delay_us = 2 * MSEC_PER_SEC;
> +	if (latency)
> +		/* Give a 50% breathing room between updates */
> +		return latency + (latency >> 1);
>   
> -		/*
> -		 * If the platform already has high transition_latency, use it
> -		 * as-is.
> -		 */
> -		if (latency > max_delay_us)
> -			return latency;
> -
> -		/*
> -		 * For platforms that can change the frequency very fast (< 2
> -		 * us), the above formula gives a decent transition delay. But
> -		 * for platforms where transition_latency is in milliseconds, it
> -		 * ends up giving unrealistic values.
> -		 *
> -		 * 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, max_delay_us);
> -	}
> -
> -	return LATENCY_MULTIPLIER;
> +	return USEC_PER_MSEC;
>   }
>   EXPORT_SYMBOL_GPL(cpufreq_policy_transition_delay_us);
>   
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index d4d2f4d1d7cb..e0e19d9c1323 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -577,12 +577,6 @@ static inline unsigned long cpufreq_scale(unsigned long old, u_int div,
>   #define CPUFREQ_POLICY_POWERSAVE	(1)
>   #define CPUFREQ_POLICY_PERFORMANCE	(2)
>   
> -/*
> - * The polling frequency depends on the capability of the processor. Default
> - * polling frequency is 1000 times the transition latency of the processor.
> - */
> -#define LATENCY_MULTIPLIER		(1000)
> -
>   struct cpufreq_governor {
>   	char	name[CPUFREQ_NAME_LEN];
>   	int	(*init)(struct cpufreq_policy *policy);