Date:	Wed, 2 Mar 2016 23:49:48 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [PATCH 6/6] cpufreq: schedutil: New governor based on scheduler
 utilization data

On Wed, Mar 2, 2016 at 6:58 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Wed, Mar 2, 2016 at 6:10 PM, Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>> Hi Rafael,
>>
>>
>> On 2 March 2016 at 03:27, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>
>>> 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.
>
> It is a formula that causes the entire available frequency range to be
> utilized proportionally to the utilization as reported by the
> scheduler (modulo the policy->min/max limits).  Its (significant IMO)
> advantage is that it doesn't require any additional factors that would
> need to be determined somehow.

In case a more formal derivation of this formula is needed, it is
based on the following 3 assumptions:

(1) Performance is a linear function of frequency.
(2) Required performance is a linear function of the utilization ratio
x = util/max as provided by the scheduler (0 <= x <= 1).
(3) The minimum possible frequency (min_freq) corresponds to x = 0 and
the maximum possible frequency (max_freq) corresponds to x = 1.

(1) and (2) combined imply that

f = a * x + b

(f - frequency, a, b - constants to be determined) and then (3) quite
trivially leads to b = min_freq and a = max_freq - min_freq.

Now, of course, the linearity assumptions may be questioned, but then
it's just the first approximation.  If you go any further, though, you
end up with an expansion series like this:

f(x) = c_0 + c_1 * x + c_2 * x^2 + c_3 * x^3 + ...

where all of the c_j need to be determined in principle.  With luck,
if you can guess what kind of a function f(x) may be, it may be
possible to reduce the number of coefficients to determine, but
question is whether or not that is going to work universally for all
systems.

Thanks,
Rafael

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