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

From: Rafael J. Wysocki
Date: Mon Nov 14 2016 - 15:44:48 EST

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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.

Thanks,
Rafael

Next message: Borislav Petkov: "[PATCH -v1.1] kbuild: Steal gcc's pie from the very beginning"
Previous message: Alexandre Belloni: "[GIT PULL] RTC fixes for 4.9"
In reply to: Viresh Kumar: "Re: [PATCH v2] cpufreq: conservative: Decrease frequency faster when the update deferred"
Next in thread: Stratos Karafotis: "Re: [PATCH v2] cpufreq: conservative: Decrease frequency faster when the update deferred"
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