Re: [PATCH v2] cpufreq: conservative: Decrease frequency faster when the update deferred

From: Rafael J. Wysocki
Date: Mon Nov 14 2016 - 17:09:49 EST

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On Mon, Nov 14, 2016 at 10:59 PM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> On Mon, Nov 14, 2016 at 10:46 PM, Stratos Karafotis
> <stratosk@xxxxxxxxxxxx> wrote:
>>
>>
>> On 14/11/2016 10:44 ÎÎ, Rafael J. Wysocki wrote:
>>> On Sat, Nov 12, 2016 at 10:04 PM, Stratos Karafotis
>>> <stratosk@xxxxxxxxxxxx> 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 to min frequency when the workload
>>>> is finished.
>>>>
>>>> If the update function 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 at every sampling
>>>> rate.
>>>>
>>>> To resolve the above issue calculate the number of sampling periods
>>>> that the update is 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@xxxxxxxxxxxx>
>>>> ---
>>>> v1 -> v2
>>>> - Use correct terminology in change log
>>>> - Change the member variable name from 'deferred_periods' to 'idle_periods'
>>>> - Fix format issue
>>>>
>>>> drivers/cpufreq/cpufreq_conservative.c | 14 +++++++++++++-
>>>> drivers/cpufreq/cpufreq_governor.c | 18 +++++++++++++-----
>>>> drivers/cpufreq/cpufreq_governor.h | 1 +
>>>> 3 files changed, 27 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
>>>> index fa5ece3..d787772 100644
>>>> --- a/drivers/cpufreq/cpufreq_conservative.c
>>>> +++ b/drivers/cpufreq/cpufreq_conservative.c
>>>> @@ -73,7 +73,19 @@ static unsigned int cs_dbs_update(struct cpufreq_policy *policy)
>>>> */
>>>> if (cs_tuners->freq_step == 0)
>>>> goto out;
>>>> -
>>>> + /*
>>>> + * Decrease requested_freq for each idle period that we didn't
>>>> + * update the frequency
>>>> + */
>>>> + if (policy_dbs->idle_periods < UINT_MAX) {
>>>> + unsigned int freq_target = policy_dbs->idle_periods *
>>>> + get_freq_target(cs_tuners, policy);
>>>> + if (requested_freq > freq_target)
>>>> + requested_freq -= freq_target;
>>>> + else
>>>> + requested_freq = policy->min;
>>>> + policy_dbs->idle_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 3729474..1bc7137 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, idle_periods = UINT_MAX;
>>>> unsigned int sampling_rate, io_busy, j;
>>>>
>>>> /*
>>>> @@ -163,8 +163,12 @@ 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 < idle_periods)
>>>> + idle_periods = periods;
>>>> +
>>>> /*
>>>> * If the CPU had gone completely idle and a task has
>>>> * just woken up on this CPU now, it would be unfair to
>>>> @@ -189,8 +193,10 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
>>>> * 'time_elapsed' (as compared to the sampling rate)
>>>> * indicates this scenario.
>>>> */
>>>> - load = j_cdbs->prev_load;
>>>> - j_cdbs->prev_load = 0;
>>>> + if (j_cdbs->prev_load) {
>>>> + load = j_cdbs->prev_load;
>>>> + j_cdbs->prev_load = 0;
>>>> + }
>>>> } else {
>>>> if (time_elapsed >= idle_time) {
>>>> load = 100 * (time_elapsed - idle_time) / time_elapsed;
>>>> @@ -218,6 +224,8 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
>>>> if (load > max_load)
>>>> max_load = load;
>>>> }
>>>> + policy_dbs->idle_periods = idle_periods;
>>>> +
>>>> return max_load;
>>>> }
>>>> EXPORT_SYMBOL_GPL(dbs_update);
>>>
>>> I have a murky suspicion that the changes in dbs_update() are going to
>>> break something. I need to recall what it was, though.
>>
>> The only change in dbs_update() is the calculation of 'idle_periods'.
>> If I don't miss something I left current functionality untouched.
>
> Well, not quite. The else branch may now trigger when
> j_cdbs->prev_load is zero too which it didn't do before, AFAICS.

What I mean is that the "if else" never triggers when
j_cdbs->prev_load is zero before the change, but that changes, so the
"else" branch will not cover the "j_cdbs->prev_load equal to zero"
case any more. I'm not sure how much that matters ATM, though.

Sent too quickly, sorry.

Thanks,
Rafael

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