Re: [PATCH 2/2] cpuidle/drivers/menu: Remove get_loadavg in the performance multiplier

[Daniel Lezcano]

On 04/10/2018 10:22, Rafael J. Wysocki wrote:
> On Thu, Oct 4, 2018 at 9:42 AM Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> wrote:
>>
>> The function get_loadavg() returns almost always zero. To be more
>> precise, statistically speaking for a total of 1023379 times passing
>> in the function, the load is equal to zero 1020728 times, greater than
>> 100, 610 times, the remaining is between 0 and 5.
>>
>> In 2011, the get_loadavg() was removed from the Android tree because
>> of the above [1]. At this time, the load was:
>>
>> unsigned long this_cpu_load(void)
>> {
>>         struct rq *this = this_rq();
>>         return this->cpu_load[0];
>> }
>>
>> In 2014, the code was changed by commit 372ba8cb46b2 (cpuidle: menu: Lookup CPU
>> runqueues less) and the load is:
>>
>> void get_iowait_load(unsigned long *nr_waiters, unsigned long *load)
>> {
>>         struct rq *rq = this_rq();
>>         *nr_waiters = atomic_read(&rq->nr_iowait);
>>         *load = rq->load.weight;
>> }
>>
>> with the same result.
>>
>> Both measurements show using the load in this code path does no matter
>> anymore. Removing it.
>>
>> [1] https://android.googlesource.com/kernel/common/+/4dedd9f124703207895777ac6e91dacde0f7cc17
>>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
>> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
>> Cc: Colin Cross <ccross@xxxxxxxxxxx>
>> Cc: Ramesh Thomas <ramesh.thomas@xxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxx>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
>> ---
>>  drivers/cpuidle/governors/menu.c | 26 +++++++-------------------
>>  include/linux/sched/stat.h       |  1 -
>>  kernel/sched/core.c              | 13 -------------
>>  3 files changed, 7 insertions(+), 33 deletions(-)
>>
>> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
>> index e26a409..066b01f 100644
>> --- a/drivers/cpuidle/governors/menu.c
>> +++ b/drivers/cpuidle/governors/menu.c
>> @@ -135,11 +135,6 @@ struct menu_device {
>>  #define LOAD_INT(x) ((x) >> FSHIFT)
>>  #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
>>
>> -static inline int get_loadavg(unsigned long load)
>> -{
>> -       return LOAD_INT(load) * 10 + LOAD_FRAC(load) / 10;
>> -}
>> -
>>  static inline int which_bucket(unsigned int duration, unsigned long nr_iowaiters)
>>  {
>>         int bucket = 0;
>> @@ -173,18 +168,10 @@ static inline int which_bucket(unsigned int duration, unsigned long nr_iowaiters
>>   * to be, the higher this multiplier, and thus the higher
>>   * the barrier to go to an expensive C state.
>>   */
>> -static inline int performance_multiplier(unsigned long nr_iowaiters, unsigned long load)
>> +static inline int performance_multiplier(unsigned long nr_iowaiters)
>>  {
>> -       int mult = 1;
>> -
>> -       /* for higher loadavg, we are more reluctant */
>> -
>> -       mult += 2 * get_loadavg(load);
>> -
>> -       /* for IO wait tasks (per cpu!) we add 5x each */
>> -       mult += 10 * nr_iowaiters;
>> -
>> -       return mult;
>> +       /* for IO wait tasks (per cpu!) we add 10x each */
>> +       return 1 + 10 * nr_iowaiters;
>>  }
>>
>>  static DEFINE_PER_CPU(struct menu_device, menu_devices);
>> @@ -290,7 +277,7 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>>         int idx;
>>         unsigned int interactivity_req;
>>         unsigned int expected_interval;
>> -       unsigned long nr_iowaiters, cpu_load;
>> +       unsigned long nr_iowaiters;
>>         ktime_t delta_next;
>>
>>         if (data->needs_update) {
>> @@ -307,7 +294,7 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>>         /* determine the expected residency time, round up */
>>         data->next_timer_us = ktime_to_us(tick_nohz_get_sleep_length(&delta_next));
>>
>> -       get_iowait_load(&nr_iowaiters, &cpu_load);
>> +       nr_iowaiters = nr_iowait_cpu(dev->cpu);
>>         data->bucket = which_bucket(data->next_timer_us, nr_iowaiters);
>>
>>         /*
>> @@ -359,7 +346,8 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>>                  * Use the performance multiplier and the user-configurable
>>                  * latency_req to determine the maximum exit latency.
>>                  */
>> -               interactivity_req = data->predicted_us / performance_multiplier(nr_iowaiters, cpu_load);
>> +               interactivity_req = data->predicted_us /
>> +                       performance_multiplier(nr_iowaiters);
> 
> I wouldn't break this line.

Ok, mind if I break the line in a separate patch before ? (my git
pre-commit hook runs checkpatch and it complains and prevent to commit
the patch because of the line length (94)).

>>                 if (latency_req > interactivity_req)
>>                         latency_req = interactivity_req;
>>         }
>> diff --git a/include/linux/sched/stat.h b/include/linux/sched/stat.h
>> index 04f1321..f30954c 100644
>> --- a/include/linux/sched/stat.h
>> +++ b/include/linux/sched/stat.h
>> @@ -20,7 +20,6 @@ extern unsigned long nr_running(void);
>>  extern bool single_task_running(void);
>>  extern unsigned long nr_iowait(void);
>>  extern unsigned long nr_iowait_cpu(int cpu);
>> -extern void get_iowait_load(unsigned long *nr_waiters, unsigned long *load);
>>
>>  static inline int sched_info_on(void)
>>  {
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index b88a145..5605f03 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2873,25 +2873,12 @@ unsigned long long nr_context_switches(void)
>>
>>         return sum;
>>  }
>> -/*
>> - * Consumers of these two interfaces, like for example the cpufreq menu
>> - * governor are using nonsensical data. Boosting frequency for a CPU that has
>> - * IO-wait which might not even end up running the task when it does become
>> - * runnable.
>> - */
> 
> Doesn't the comment still apply to nr_iowait_cpu()?

The comment is very confusing. I talks about a cpufreq menu governor and
boosting frequency. I don't see this function used in the cpufreq
framework but in:

kernel/time/tick-sched.c
fs/proc/stat.c
drivers/cpuidle/governors/menu.c

The comment is irrelevant as the remaining function is used for
statistics in addition to the perf multiplier. It does exactly what the
function name is.

>>  unsigned long nr_iowait_cpu(int cpu)
>>  {
>>         return atomic_read(&cpu_rq(cpu)->nr_iowait);
>>  }
>>
>> -void get_iowait_load(unsigned long *nr_waiters, unsigned long *load)
>> -{
>> -       struct rq *rq = this_rq();
>> -       *nr_waiters = atomic_read(&rq->nr_iowait);
>> -       *load = rq->load.weight;
>> -}
>> -
>>  /*
>>   * IO-wait accounting, and how its mostly bollocks (on SMP).
>>   *
>> --


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