linux-kernel - Re: [PATCH v2 3/3] cpufreq: schedutil: map raw required frequency to driver frequency

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Date:	Thu, 26 May 2016 12:46:29 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH v2 3/3] cpufreq: schedutil: map raw required frequency to
 driver frequency

On 25-05-16, 19:53, Steve Muckle wrote:
> The slow-path frequency transition path is relatively expensive as it
> requires waking up a thread to do work. Should support be added for
> remote CPU cpufreq updates that is also expensive since it requires an
> IPI. These activities should be avoided if they are not necessary.
> 
> To that end, calculate the actual driver-supported frequency required by
> the new utilization value in schedutil by using the recently added
> cpufreq_driver_resolve_freq callback. If it is the same as the
> previously requested driver frequency then there is no need to continue
> with the update assuming the cpu frequency limits have not changed. This
> will have additional benefits should the semantics of the rate limit be
> changed to apply solely to frequency transitions rather than to
> frequency calculations in schedutil.

Maybe mention here that this patch isn't avoiding those IPIs yet, I
got an impression earlier that they are avoided with it.

> The last raw required frequency is cached. This allows the driver
> frequency lookup to be skipped in the event that the new raw required
> frequency matches the last one, assuming a frequency update has not been
> forced due to limits changing (indicated by a next_freq value of
> UINT_MAX, see sugov_should_update_freq).

I am not sure this is required, see below..

> Signed-off-by: Steve Muckle <smuckle@...aro.org>
> ---
>  kernel/sched/cpufreq_schedutil.c | 30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 154ae3a51e86..ef73ca4d8d78 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -45,6 +45,8 @@ struct sugov_cpu {
>  	struct update_util_data update_util;
>  	struct sugov_policy *sg_policy;
>  
> +	unsigned int cached_raw_freq;
> +
>  	/* The fields below are only needed when sharing a policy. */
>  	unsigned long util;
>  	unsigned long max;
> @@ -104,7 +106,7 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>  
>  /**
>   * get_next_freq - Compute a new frequency for a given cpufreq policy.
> - * @policy: cpufreq policy object to compute the new frequency for.
> + * @sg_cpu: schedutil cpu object to compute the new frequency for.
>   * @util: Current CPU utilization.
>   * @max: CPU capacity.
>   *
> @@ -119,14 +121,24 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
>   * next_freq = C * curr_freq * util_raw / max
>   *
>   * Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
> + *
> + * The lowest driver-supported frequency which is equal or greater than the raw
> + * next_freq (as calculated above) is returned.
>   */
> -static unsigned int get_next_freq(struct cpufreq_policy *policy,
> -				  unsigned long util, unsigned long max)
> +static unsigned int get_next_freq(struct sugov_cpu *sg_cpu, unsigned long util,
> +				  unsigned long max)
>  {
> +	struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> +	struct cpufreq_policy *policy = sg_policy->policy;
>  	unsigned int freq = arch_scale_freq_invariant() ?
>  				policy->cpuinfo.max_freq : policy->cur;
>  
> -	return (freq + (freq >> 2)) * util / max;
> +	freq = (freq + (freq >> 2)) * util / max;
> +
> +	if (freq == sg_cpu->cached_raw_freq && sg_policy->next_freq != UINT_MAX)
> +		return sg_policy->next_freq;

I am not sure what the benefit of the second comparison to UINT_MAX
is. The output of below code will be same if the freq was ==
cached_raw_freq, no matter what.

I also have a doubt (I am quite sure Rafael will have a reason for
that, which I am failing to understand now), on why we are doing
next_freq == UINT_MAX in sugov_should_update_freq().

I understand that because the limits might have changed,
need_freq_update would have been set to true. We should evaluate
next-freq again without worrying about the load or the time since last
evaluation.

But what will happen by forcefully calling the cpufreq routines to
change the frequency, if next_freq hasn't changed even after limits
updates? Wouldn't that call always return early because the new freq
and the current freq are going to be same ?

@Rafael: Sorry for asking this so late :(

-- 
viresh