Re: [PATCH v1 2/4] cpufreq: schedutil: Adjust utilization instead of frequency

Viresh Kumar
2020-12-08T08:52:52Z

On 07-12-20, 17:29, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> When avoiding reduction of the frequency after the target CPU has
> been busy since the previous frequency update, adjust the utilization
> instead of adjusting the frequency, because doing so is more prudent
> (it is done to counter a possible utilization deficit after all) and
> it will allow some code to be shared after a subsequent change.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> kernel/sched/cpufreq_schedutil.c | 11 ++++-------
> 1 file changed, 4 insertions(+), 7 deletions(-)
>
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -437,7 +437,7 @@ static void sugov_update_single(struct u
> {
> struct sugov_cpu *sg_cpu = container_of(hook, struct sugov_cpu, update_util);
> struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> - unsigned int cached_freq = sg_policy->cached_raw_freq;
> + unsigned long prev_util = sg_cpu->util;
> unsigned int next_f;
>
> sugov_iowait_boost(sg_cpu, time, flags);
> @@ -451,17 +451,14 @@ static void sugov_update_single(struct u
> sugov_get_util(sg_cpu);
> sugov_iowait_apply(sg_cpu, time);
>
> - next_f = get_next_freq(sg_policy, sg_cpu->util, sg_cpu->max);
> /*
> * Do not reduce the frequency if the CPU has not been idle
> * recently, as the reduction is likely to be premature then.
> */
> - if (sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> - next_f = sg_policy->next_freq;
> + if (sugov_cpu_is_busy(sg_cpu) && sg_cpu->util < prev_util)
> + sg_cpu->util = prev_util;
>
> - /* Restore cached freq as next_freq has changed */
> - sg_policy->cached_raw_freq = cached_freq;
> - }
> + next_f = get_next_freq(sg_policy, sg_cpu->util, sg_cpu->max);

I don't think we can replace freq comparison by util, or at least it will give
us a different final frequency and the behavior is changed.

Lets take an example, lets say current freq is 1 GHz and max is 1024.

Round 1: Lets say util is 1000

next_f = 1GHz * 1.25 * 1000/1024 = 1.2 GHz

Round 2: Lets say util has come down to 900 here,

before the patch:

next_f = 1.2 GHz * 1.25 * 900/1024 = 1.31 GHz

after the patch:

next_f = 1.2 GHz * 1.25 * 1000/1024 = 1.45 GHz

Or did I make a mistake here ?

--
viresh