Re: [Resend][PATCH] cpufreq: conservative: Decrease frequency faster when the timer deferred

Stratos Karafotis
2016-11-07T17:28:06Z

Hi,

Thanks for reviewing.

On 07/11/2016 08:12 πμ, Viresh Kumar wrote:
> For the record, I have never got the original mail with this subject.

I'm sorry for inconvenience. It seems that I had an issue on my mail
server.

> On 06-11-16, 11:19, Stratos Karafotis wrote:
>> Conservative governor changes the CPU frequency in steps.
>> That means that if a CPU runs at max frequency, it will need several
>> sampling periods to return at min frequency when the workload
>> is finished.
>>
>> If the timer that calculates the load and target frequency is deferred,
>> the governor might need even more time to decrease the frequency.
>>
>> This may have impact to power consumption and after all conservative
>> should decrease the frequency if there is no workload every sampling
>> rate.
>>
>> To resolve the above issue calculate the number of sampling periods
>> that the timer deferred. Considering that for each sampling period
>> conservative should drop the frequency by a freq_step because the
>> CPU was idle apply the proper subtraction to requested frequency.
>>
>> Below, the kernel trace with and without this patch. First an
>> intensive workload is applied on a specific CPU. Then the workload
>> is removed and the CPU goes to idle.
>>
>> WITHOUT
>> -------
>> <idle>-0 [007] dN.. 620.329153: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 620.350857: cpu_frequency: state=1700000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.370856: cpu_frequency: state=1900000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.390854: cpu_frequency: state=2100000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.411853: cpu_frequency: state=2200000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.432854: cpu_frequency: state=2400000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.453854: cpu_frequency: state=2600000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.494856: cpu_frequency: state=2900000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.515856: cpu_frequency: state=3100000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.536858: cpu_frequency: state=3300000 cpu_id=7
>> kworker/7:2-556 [007] .... 620.557857: cpu_frequency: state=3401000 cpu_id=7
>> <idle>-0 [007] d... 669.591363: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 669.591939: cpu_idle: state=4294967295 cpu_id=7
>> <idle>-0 [007] d... 669.591980: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] dN.. 669.591989: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 670.201224: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 670.221975: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 670.222016: cpu_frequency: state=3300000 cpu_id=7
>> <idle>-0 [007] d... 670.222026: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 670.234964: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 670.801251: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 671.236046: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 671.236073: cpu_frequency: state=3100000 cpu_id=7
>> <idle>-0 [007] d... 671.236112: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 671.393437: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 671.401277: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 671.404083: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 671.404111: cpu_frequency: state=2900000 cpu_id=7
>> <idle>-0 [007] d... 671.404125: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 671.404974: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 671.501180: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 671.995414: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 671.995459: cpu_frequency: state=2800000 cpu_id=7
>> <idle>-0 [007] d... 671.995469: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 671.996287: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 672.001305: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.078374: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 672.078410: cpu_frequency: state=2600000 cpu_id=7
>> <idle>-0 [007] d... 672.078419: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.158020: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 672.158040: cpu_frequency: state=2400000 cpu_id=7
>> <idle>-0 [007] d... 672.158044: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.160038: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 672.234557: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.237121: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 672.237174: cpu_frequency: state=2100000 cpu_id=7
>> <idle>-0 [007] d... 672.237186: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.237778: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 672.267902: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.269860: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 672.269906: cpu_frequency: state=1900000 cpu_id=7
>> <idle>-0 [007] d... 672.269914: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.271902: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 672.751342: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 672.823056: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-556 [007] .... 672.823095: cpu_frequency: state=1600000 cpu_id=7
>>
>> WITH
>> ----
>> <idle>-0 [007] dN.. 4380.928009: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-399 [007] .... 4380.949767: cpu_frequency: state=2000000 cpu_id=7
>> kworker/7:2-399 [007] .... 4380.969765: cpu_frequency: state=2200000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.009766: cpu_frequency: state=2500000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.029767: cpu_frequency: state=2600000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.049769: cpu_frequency: state=2800000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.069769: cpu_frequency: state=3000000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.089771: cpu_frequency: state=3100000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.109772: cpu_frequency: state=3400000 cpu_id=7
>> kworker/7:2-399 [007] .... 4381.129773: cpu_frequency: state=3401000 cpu_id=7
>> <idle>-0 [007] d... 4428.226159: cpu_idle: state=1 cpu_id=7
>> <idle>-0 [007] d... 4428.226176: cpu_idle: state=4294967295 cpu_id=7
>> <idle>-0 [007] d... 4428.226181: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 4428.227177: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 4428.551640: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 4428.649239: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-399 [007] .... 4428.649268: cpu_frequency: state=2800000 cpu_id=7
>> <idle>-0 [007] d... 4428.649278: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 4428.689856: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 4428.799542: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 4428.801683: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-399 [007] .... 4428.801748: cpu_frequency: state=1700000 cpu_id=7
>> <idle>-0 [007] d... 4428.801761: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 4428.806545: cpu_idle: state=4294967295 cpu_id=7
>> ...
>> <idle>-0 [007] d... 4429.051880: cpu_idle: state=4 cpu_id=7
>> <idle>-0 [007] d... 4429.086240: cpu_idle: state=4294967295 cpu_id=7
>> kworker/7:2-399 [007] .... 4429.086293: cpu_frequency: state=1600000 cpu_id=7
>>
>> Without the patch the CPU dropped to min frequency after 3.2s
>> With the patch applied the CPU dropped to min frequency after 0.86s
>>
>> Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>
>> ---
>> drivers/cpufreq/cpufreq_conservative.c | 9 +++++++++
>> drivers/cpufreq/cpufreq_governor.c | 18 +++++++++++++-----
>> drivers/cpufreq/cpufreq_governor.h | 1 +
>> 3 files changed, 23 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
>> index 1347589..07dac72 100644
>> --- a/drivers/cpufreq/cpufreq_conservative.c
>> +++ b/drivers/cpufreq/cpufreq_conservative.c
>> @@ -74,6 +74,15 @@ static unsigned int cs_dbs_timer(struct cpufreq_policy *policy)
>> if (cs_tuners->freq_step == 0)
>> goto out;
>>
>> + if (policy_dbs->deferred_periods < UINT_MAX) {
>
> Perhaps this all should be done only if we are going to decrease the frequency,
> i.e. somewhere down than what you are proposing.

Yes, it could be done only when we decrease frequency. But I thought that maybe
this is against conservative governor principle.

I initially observed this issue on a Snapdragon 808 using conservative on the
big cluster (A57). The CPU seemed to remain in high frequencies for
long time (even 10 seconds) before it returns to min.

So, most probably the load after the deferred period is completely unrelated to
the previous one. If we apply this heuristic only when the frequency will be
decreased (and having in mind that we copy the load value from the previous
period), IMHO I'm afraid that the conservative will be still more aggressive even
from ondemand governor.

>> + unsigned int freq_target = policy_dbs->deferred_periods *
>> + get_freq_target(cs_tuners, policy);
>> + if (requested_freq > freq_target)
>> + requested_freq -= freq_target;
>> + else
>> + requested_freq = policy->min;
>> + policy_dbs->deferred_periods = UINT_MAX;
>> + }
>> /*
>> * If requested_freq is out of range, it is likely that the limits
>> * changed in the meantime, so fall back to current frequency in that
>> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
>> index 642dd0f..0d498a0 100644
>> --- a/drivers/cpufreq/cpufreq_governor.c
>> +++ b/drivers/cpufreq/cpufreq_governor.c
>> @@ -117,7 +117,7 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
>> struct policy_dbs_info *policy_dbs = policy->governor_data;
>> struct dbs_data *dbs_data = policy_dbs->dbs_data;
>> unsigned int ignore_nice = dbs_data->ignore_nice_load;
>> - unsigned int max_load = 0;
>> + unsigned int max_load = 0, deferred_periods = UINT_MAX;
>> unsigned int sampling_rate, io_busy, j;
>>
>> /*
>> @@ -163,8 +163,13 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
>> * calls, so the previous load value can be used then.
>> */
>> load = j_cdbs->prev_load;
>> - } else if (unlikely(time_elapsed > 2 * sampling_rate &&
>> - j_cdbs->prev_load)) {
>> + } else if (unlikely(time_elapsed > 2 * sampling_rate)) {
>> + unsigned int periods = time_elapsed / sampling_rate;
>> +
>> + if (periods < deferred_periods)
>> + deferred_periods = periods;
>> +
>> + if (j_cdbs->prev_load) {
>> /*
>
> You forgot to shift the below comment by a tab. Maybe just position the above
> 'if' statement after the comment.

OK, I will place the 'if' statement after the comment, because the shifting
will cause the block to exceed the 80 columns limit.

Regards,
Stratos