Re: [PATCH 4/5] cpufreq: schedutil: map raw required frequency to CPU-supported frequency

[Rafael J. Wysocki]

On Mon, May 9, 2016 at 11:20 PM, Steve Muckle <steve.muckle@xxxxxxxxxx> wrote:
> The mechanisms for remote CPU updates and slow-path frequency
> transitions are relatively expensive - the former is an IPI while the
> latter requires waking up a thread to do work. These activities should
> be avoided if they are not necessary. To that end, calculate the
> actual target-supported frequency required by the new utilization
> value in schedutil. If it is the same as the previously requested
> frequency then there is no need to continue with the update.

Unless the max/min limits changed in the meantime, right?

>
> Signed-off-by: Steve Muckle <smuckle@xxxxxxxxxx>
> ---
>  kernel/sched/cpufreq_schedutil.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 6cb2ecc204ec..e185075fcb5c 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -153,14 +153,26 @@ static void sugov_update_commit(struct sugov_cpu *sg_cpu, int cpu, 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 target-supported frequency which is equal or greater than the raw
> + * next_freq (as calculated above) is returned, or the CPU's max_freq if such
> + * a target-supported frequency does not exist.
>   */
>  static unsigned int get_next_freq(struct cpufreq_policy *policy,
>                                   unsigned long util, unsigned long max)
>  {
> +       struct cpufreq_frequency_table *entry;
>         unsigned int freq = arch_scale_freq_invariant() ?
>                                 policy->cpuinfo.max_freq : policy->cur;
> +       unsigned int target_freq = UINT_MAX;
> +
> +       freq = (freq + (freq >> 2)) * util / max;
> +
> +       cpufreq_for_each_valid_entry(entry, policy->freq_table)
> +               if (entry->frequency >= freq && entry->frequency < target_freq)
> +                       target_freq = entry->frequency;

Please don't assume that every driver will have a frequency table.
That may not be the case in the future (and I'm not even sure about
the existing CPPC driver for that matter).

>
> -       return (freq + (freq >> 2)) * util / max;
> +       return target_freq != UINT_MAX ? target_freq : policy->cpuinfo.max_freq;
>  }
>
>  static void sugov_update_single(struct update_util_data *hook, u64 time,
> --
> 2.4.10
>