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Date: Thu, 4 Oct 2018 10:40:15 +0200
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>,
Linux PM <linux-pm@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Todd Kjos <tkjos@...gle.com>,
Joel Fernandes <joelaf@...gle.com>,
Colin Cross <ccross@...roid.com>,
Ramesh Thomas <ramesh.thomas@...el.com>,
Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
Rafael Wysocki <rafael.j.wysocki@...el.com>,
Alex Shi <alex.shi@...aro.org>,
Philippe Ombredanne <pombredanne@...b.com>,
Thomas Gleixner <tglx@...utronix.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Kate Stewart <kstewart@...uxfoundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] cpuidle/drivers/menu: Remove get_loadavg in the
performance multiplier
On 04/10/2018 10:22, Rafael J. Wysocki wrote:
> On Thu, Oct 4, 2018 at 9:42 AM Daniel Lezcano <daniel.lezcano@...aro.org> 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@...radead.org>
>> Cc: Todd Kjos <tkjos@...gle.com>
>> Cc: Joel Fernandes <joelaf@...gle.com>
>> Cc: Colin Cross <ccross@...roid.com>
>> Cc: Ramesh Thomas <ramesh.thomas@...el.com>
>> Cc: Mel Gorman <mgorman@...e.de>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>> ---
>> 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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