Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler utilization data

[Vincent Guittot]

Hi Rafael,


On 2 March 2016 at 03:27, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> Add a new cpufreq scaling governor, called "schedutil", that uses
> scheduler-provided CPU utilization information as input for making
> its decisions.
>
> Doing that is possible after commit fe7034338ba0 (cpufreq: Add
> mechanism for registering utilization update callbacks) that
> introduced cpufreq_update_util() called by the scheduler on
> utilization changes (from CFS) and RT/DL task status updates.
> In particular, CPU frequency scaling decisions may be based on
> the the utilization data passed to cpufreq_update_util() by CFS.
>
> The new governor is relatively simple.
>
> The frequency selection formula used by it is essentially the same
> as the one used by the "ondemand" governor, although it doesn't use
> the additional up_threshold parameter, but instead of computing the
> load as the "non-idle CPU time" to "total CPU time" ratio, it takes
> the utilization data provided by CFS as input.  More specifically,
> it represents "load" as the util/max ratio, where util and max
> are the utilization and CPU capacity coming from CFS.
>

[snip]

> +
> +static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
> +                               unsigned long util, unsigned long max,
> +                               unsigned int next_freq)
> +{
> +       struct cpufreq_policy *policy = sg_policy->policy;
> +       unsigned int rel;
> +
> +       if (next_freq > policy->max)
> +               next_freq = policy->max;
> +       else if (next_freq < policy->min)
> +               next_freq = policy->min;
> +
> +       sg_policy->last_freq_update_time = time;
> +       if (sg_policy->next_freq == next_freq)
> +               return;
> +
> +       sg_policy->next_freq = next_freq;
> +       /*
> +        * If utilization is less than max / 4, use RELATION_C to allow the
> +        * minimum frequency to be selected more often in case the distance from
> +        * it to the next available frequency in the table is significant.
> +        */
> +       rel = util < (max >> 2) ? CPUFREQ_RELATION_C : CPUFREQ_RELATION_L;
> +       if (policy->fast_switch_possible) {
> +               cpufreq_driver_fast_switch(policy, next_freq, rel);
> +       } else {
> +               sg_policy->relation = rel;
> +               sg_policy->work_in_progress = true;
> +               irq_work_queue(&sg_policy->irq_work);
> +       }
> +}
> +
> +static void sugov_update_single(struct update_util_data *data, u64 time,
> +                               unsigned long util, unsigned long max)
> +{
> +       struct sugov_cpu *sg_cpu = container_of(data, struct sugov_cpu, update_util);
> +       struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> +       unsigned int min_f, max_f, next_f;
> +
> +       if (!sugov_should_update_freq(sg_policy, time))
> +               return;
> +
> +       min_f = sg_policy->policy->cpuinfo.min_freq;
> +       max_f = sg_policy->policy->cpuinfo.max_freq;
> +       next_f = util > max ? max_f : min_f + util * (max_f - min_f) / max;

I think it has been pointed out in another email's thread but you
should change the way the next_f is computed. util reflects the
utilization of a CPU from 0 to its max compute capacity whereas
ondemand was using the load at the current frequency during the last
time window. I have understood that you want to keep same formula than
ondemand as a starting point but you use a different input to
calculate the next frequency so i don't see the rational of keeping
this formula. Saying that, even the simple formula next_f = util > max
? max_f : util * (max_f) / max will not work properly if the frequency
invariance is enable because the utilization becomes capped by the
current compute capacity so next_f will never be higher than current
freq (unless a task move on the rq).  That was one reason of using a
threshold in sched-freq proposal (and there are on going dev to try to
solve this limitation).
IIIUC, frequency invariance is not enable on your platform so you have
not seen the problem but you have probably see that selection of your
next_f was not really stable. Without frequency invariance, the
utilization will be overestimated when running at lower frequency so
the governor will probably select a frequency that is higher than
necessary but then the utilization will decrease at this higher
frequency so the governor will probably decrease the frequency and so
on until you found the right frequency that will generate the right
utilisation value

Regards,
Vincent

> +
> +       sugov_update_commit(sg_policy, time, util, max, next_f);
> +}
> +
> +static unsigned int sugov_next_freq(struct sugov_policy *sg_policy,
> +                                   unsigned long util, unsigned long max)
> +{
> +       struct cpufreq_policy *policy = sg_policy->policy;
> +       unsigned int min_f = policy->cpuinfo.min_freq;
> +       unsigned int max_f = policy->cpuinfo.max_freq;
> +       u64 last_freq_update_time = sg_policy->last_freq_update_time;
> +       unsigned int j;
> +
> +       if (util > max)
> +               return max_f;
> +
> +       for_each_cpu(j, policy->cpus) {
> +               struct sugov_cpu *j_sg_cpu;
> +               unsigned long j_util, j_max;
> +               u64 delta_ns;
> +
> +               if (j == smp_processor_id())
> +                       continue;
> +
> +               j_sg_cpu = &per_cpu(sugov_cpu, j);
> +               /*
> +                * If the CPU utilization was last updated before the previous
> +                * frequency update and the time elapsed between the last update
> +                * of the CPU utilization and the last frequency update is long
> +                * enough, don't take the CPU into account as it probably is
> +                * idle now.
> +                */
> +               delta_ns = last_freq_update_time - j_sg_cpu->last_update;
> +               if ((s64)delta_ns > NSEC_PER_SEC / HZ)
> +                       continue;
> +
> +               j_util = j_sg_cpu->util;
> +               j_max = j_sg_cpu->max;
> +               if (j_util > j_max)
> +                       return max_f;
> +
> +               if (j_util * max > j_max * util) {
> +                       util = j_util;
> +                       max = j_max;
> +               }
> +       }
> +
> +       return min_f + util * (max_f - min_f) / max;
> +}
> +

[snip]