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Message-ID: <4EBA6046.2060305@linux.vnet.ibm.com>
Date: Wed, 09 Nov 2011 16:43:10 +0530
From: Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>
To: Kevin Hilman <khilman@...com>
CC: linux-kernel@...r.kernel.org,
Arjan van de Ven <arjan@...radead.org>,
linux-arm-kernel@...ts.infradead.org, linux-omap@...r.kernel.org,
linux-pm@...ts.linux-foundation.org,
Nicole Chalhoub <n-chalhoub@...com>
Subject: Re: [PATCH] CPUIdle: Reevaluate C-states under CPU load to favor
deeper C-states
On Saturday 05 November 2011 03:16 AM, Kevin Hilman wrote:
> ping v2
>
> Kevin Hilman <khilman@...com> writes:
>
>> From: Nicole Chalhoub <n-chalhoub@...com>
>>
>> While there is CPU load, program a C-state specific one-shot timer in
>> order to give CPUidle another opportunity to pick a deeper C-state
>> instead of spending potentially long idle times in a shallow C-state.
>>
>> Long winded version:
>> When going idle with a high load average, CPUidle menu governor will
>> decide to pick a shallow C-state since one of the guiding principles
>> of the menu governor is "The busier the system, the less impact of
>> C-states is acceptable" (taken from cpuidle/governors/menu.c.)
>> That makes perfect sense.
>>
>> However, there are missed power-saving opportunities for bursty
>> workloads with long idle times (e.g. MP3 playback.) Given such a
>> workload, because of the load average, CPUidle tends to pick a shallow
>> C-state. Because we also go tickless, this shallow C-state is used
>> for the duration of the idle period. If the idle period is long, a
>> deeper C state would've resulted in better power savings.
>> This patch provides an additional opportuntity for CPUidle to pick a
>> deeper C-state by programming a timer (with a C-state specific timeout)
>> such that the CPUidle governor will have another opportunity to pick a
>> deeper C-state.
>>
>> Adding this timer for C-state reevaluation improved the load estimation
>> on our ARM/OMAP4 platform and increased the time spent in deep C-states
>> (~50% of idle time in C-states deeper than C1). A power saving of ~10mA
>> at battery level is observed during MP3 playback on OMAP4/Blaze board.
>>
>> Signed-off-by: Nicole Chalhoub <n-chalhoub@...com>
>> Signed-off-by: Kevin Hilman <khilman@...com>
>> ---
>> drivers/cpuidle/cpuidle.c | 28 +++++++++++++++++++++++++-
>> drivers/cpuidle/governors/menu.c | 39 ++++++++++++++++++++++++++++++++-----
>> include/linux/cpuidle.h | 4 +++
>> 3 files changed, 63 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
>> index 1994885..4b1ac0c 100644
>> --- a/drivers/cpuidle/cpuidle.c
>> +++ b/drivers/cpuidle/cpuidle.c
>> @@ -92,13 +92,33 @@ static void cpuidle_idle_call(void)
>> target_state->time += (unsigned long long)dev->last_residency;
>> target_state->usage++;
>>
>> - /* give the governor an opportunity to reflect on the outcome */
>> - if (cpuidle_curr_governor->reflect)
>> + hrtimer_cancel(&dev->cstate_timer);
>> +
>> + /*
>> + * Give the governor an opportunity to reflect on the outcome
>> + * Do not take into account the wakeups due to the hrtimer, they
>> + * should not impact the predicted idle time.
>> + */
>> + if ((!dev->hrtimer_expired) && cpuidle_curr_governor->reflect)
>> cpuidle_curr_governor->reflect(dev);
>> trace_power_end(0);
>> }
>>
>> /**
>> + * cstate_reassessment_timer - interrupt handler of the cstate hrtimer
>> + * @handle: the expired hrtimer
>> + */
>> +static enum hrtimer_restart cstate_reassessment_timer(struct hrtimer *handle)
>> +{
>> + struct cpuidle_device *data =
>> + container_of(handle, struct cpuidle_device, cstate_timer);
>> +
>> + data->hrtimer_expired = 1;
>> +
>> + return HRTIMER_NORESTART;
>> +}
>> +
>> +/**
>> * cpuidle_install_idle_handler - installs the cpuidle idle loop handler
>> */
>> void cpuidle_install_idle_handler(void)
>> @@ -185,6 +205,10 @@ int cpuidle_enable_device(struct cpuidle_device *dev)
>>
>> dev->enabled = 1;
>>
>> + dev->hrtimer_expired = 0;
>> + hrtimer_init(&dev->cstate_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>> + dev->cstate_timer.function = cstate_reassessment_timer;
>> +
>> enabled_devices++;
>> return 0;
>>
>> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
>> index 1b12870..fd54584 100644
>> --- a/drivers/cpuidle/governors/menu.c
>> +++ b/drivers/cpuidle/governors/menu.c
>> @@ -125,10 +125,21 @@ struct menu_device {
>> #define LOAD_INT(x) ((x) >> FSHIFT)
>> #define LOAD_FRAC(x) LOAD_INT(((x) & (FIXED_1-1)) * 100)
>>
>> -static int get_loadavg(void)
>> +static int get_loadavg(struct cpuidle_device *dev)
>> {
>> - unsigned long this = this_cpu_load();
>> + unsigned long this;
>>
>> + /*
>> + * this_cpu_load() returns the value of rq->load.weight
>> + * at the previous scheduler tick and not the current value.
>> + * If the timer expired, that means we are in idle,there
>> + * are no more runnable processes in the current queue
>> + * =>return the current value of rq->load.weight which is 0.
>> + */
>> + if (dev->hrtimer_expired == 1)
>> + return 0;
>> + else
>> + this = this_cpu_load();
>>
>> return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10;
>> }
>> @@ -166,13 +177,13 @@ static inline int which_bucket(unsigned int duration)
>> * to be, the higher this multiplier, and thus the higher
>> * the barrier to go to an expensive C state.
>> */
>> -static inline int performance_multiplier(void)
>> +static inline int performance_multiplier(struct cpuidle_device *dev)
>> {
>> int mult = 1;
>>
>> /* for higher loadavg, we are more reluctant */
>>
>> - mult += 2 * get_loadavg();
>> + mult += 2 * get_loadavg(dev);
>>
>> /* for IO wait tasks (per cpu!) we add 5x each */
>> mult += 10 * nr_iowait_cpu(smp_processor_id());
>> @@ -236,6 +247,7 @@ static int menu_select(struct cpuidle_device *dev)
>> int latency_req = pm_qos_request(PM_QOS_CPU_DMA_LATENCY);
>> int i;
>> int multiplier;
>> + ktime_t timeout;
>>
>> if (data->needs_update) {
>> menu_update(dev);
>> @@ -256,7 +268,7 @@ static int menu_select(struct cpuidle_device *dev)
>>
>> data->bucket = which_bucket(data->expected_us);
>>
>> - multiplier = performance_multiplier();
>> + multiplier = performance_multiplier(dev);
>>
>> /*
>> * if the correction factor is 0 (eg first time init or cpu hotplug
>> @@ -287,12 +299,27 @@ static int menu_select(struct cpuidle_device *dev)
>> break;
>> if (s->exit_latency > latency_req)
>> break;
>> - if (s->exit_latency * multiplier > data->predicted_us)
>> + if (s->exit_latency * multiplier > data->predicted_us) {
>> + /*
>> + * Could not enter the next C-state because of a high
>> + * load. Set a timer in order to check the load again
>> + * after the timeout expires and re-evaluate cstate.
>> + */
>> + if (s->hrtimer_timeout != 0 && get_loadavg(dev)) {
>> + timeout =
>> + ktime_set(0,
>> + s->hrtimer_timeout * NSEC_PER_USEC);
>> + hrtimer_start(&dev->cstate_timer, timeout,
>> + HRTIMER_MODE_REL);
>> + }
>> break;
>> + }
>> data->exit_us = s->exit_latency;
>> data->last_state_idx = i;
>> }
>>
>> + /* Reset hrtimer_expired which is set when the hrtimer fires */
>> + dev->hrtimer_expired = 0;
>> return data->last_state_idx;
>> }
>>
>> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
>> index 55215cc..8d11b52 100644
>> --- a/include/linux/cpuidle.h
>> +++ b/include/linux/cpuidle.h
>> @@ -16,6 +16,7 @@
>> #include <linux/module.h>
>> #include <linux/kobject.h>
>> #include <linux/completion.h>
>> +#include <linux/hrtimer.h>
>>
>> #define CPUIDLE_STATE_MAX 8
>> #define CPUIDLE_NAME_LEN 16
>> @@ -37,6 +38,7 @@ struct cpuidle_state {
>> unsigned int exit_latency; /* in US */
>> unsigned int power_usage; /* in mW */
>> unsigned int target_residency; /* in US */
>> + unsigned int hrtimer_timeout; /* in US */
>>
>> unsigned long long usage;
>> unsigned long long time; /* in US */
>> @@ -97,6 +99,8 @@ struct cpuidle_device {
>> struct completion kobj_unregister;
>> void *governor_data;
>> struct cpuidle_state *safe_state;
>> + struct hrtimer cstate_timer;
>> + unsigned int hrtimer_expired;
>> };
>>
>> DECLARE_PER_CPU(struct cpuidle_device *, cpuidle_devices);
> --
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By setting timers when we enter non-deepest C-state possible, such that when it fires we
can re-evaluate and try moving into deeper and deeper C-states enhancing the
power savings is a good feature to have.
Looking at the current implementation, is it possible to have it as configurable option
where one can enable/disable this functionality through the backhand driver ?
Also I am thinking, instead of having them in governor
wouldnt it be a good idea to have it implemented in
the backhand driver itself ?
--Deepthi
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