[PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

Rafael J. Wysocki
2015-12-08T00:09:56Z

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Use the observation that if skip_work in struct cpu_common_dbs_info
is an atomic_t variable, the code may be rearranged to avoid using
the timer_lock spinlock in which case that lock is not necessary any
more.

Make that change and drop timer_lock.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

This is on top of my current linux-next branch. Completely experimental and
untested.

---
drivers/cpufreq/cpufreq_governor.c | 29 +++++++----------------------
drivers/cpufreq/cpufreq_governor.h | 9 ++-------
2 files changed, 9 insertions(+), 29 deletions(-)

Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
+++ linux-pm/drivers/cpufreq/cpufreq_governor.c
@@ -186,22 +186,15 @@ static inline void gov_cancel_timers(str

void gov_cancel_work(struct cpu_common_dbs_info *shared)
{
- unsigned long flags;
-
/*
* No work will be queued from timer handlers after skip_work is
* updated. And so we can safely cancel the work first and then the
* timers.
*/
- spin_lock_irqsave(&shared->timer_lock, flags);
- shared->skip_work++;
- spin_unlock_irqrestore(&shared->timer_lock, flags);
-
+ atomic_inc(&shared->skip_work);
cancel_work_sync(&shared->work);
-
gov_cancel_timers(shared->policy);
-
- shared->skip_work = 0;
+ atomic_set(&shared->skip_work, 0);
}

/* Will return if we need to evaluate cpu load again or not */
@@ -229,7 +222,6 @@ static void dbs_work_handler(struct work
struct cpufreq_policy *policy;
struct dbs_data *dbs_data;
unsigned int sampling_rate, delay;
- unsigned long flags;
bool eval_load;

policy = shared->policy;
@@ -258,9 +250,7 @@ static void dbs_work_handler(struct work
delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
mutex_unlock(&shared->timer_mutex);

- spin_lock_irqsave(&shared->timer_lock, flags);
- shared->skip_work--;
- spin_unlock_irqrestore(&shared->timer_lock, flags);
+ atomic_dec(&shared->skip_work);

gov_add_timers(policy, delay);
}
@@ -269,9 +259,6 @@ static void dbs_timer_handler(unsigned l
{
struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
struct cpu_common_dbs_info *shared = cdbs->shared;
- unsigned long flags;
-
- spin_lock_irqsave(&shared->timer_lock, flags);

/*
* Timer handler isn't allowed to queue work at the moment, because:
@@ -279,12 +266,10 @@ static void dbs_timer_handler(unsigned l
* - We are stopping the governor
* - Or we are updating the sampling rate of ondemand governor
*/
- if (!shared->skip_work) {
- shared->skip_work++;
+ if (atomic_inc_return(&shared->skip_work) > 1)
+ atomic_dec(&shared->skip_work);
+ else
queue_work(system_wq, &shared->work);
- }
-
- spin_unlock_irqrestore(&shared->timer_lock, flags);
}

static void set_sampling_rate(struct dbs_data *dbs_data,
@@ -315,7 +300,7 @@ static int alloc_common_dbs_info(struct
cdata->get_cpu_cdbs(j)->shared = shared;

mutex_init(&shared->timer_mutex);
- spin_lock_init(&shared->timer_lock);
+ atomic_set(&shared->skip_work, 0);
INIT_WORK(&shared->work, dbs_work_handler);
return 0;
}
Index: linux-pm/drivers/cpufreq/cpufreq_governor.h
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.h
+++ linux-pm/drivers/cpufreq/cpufreq_governor.h
@@ -17,6 +17,7 @@
#ifndef _CPUFREQ_GOVERNOR_H
#define _CPUFREQ_GOVERNOR_H

+#include <linux/atomic.h>
#include <linux/cpufreq.h>
#include <linux/kernel_stat.h>
#include <linux/module.h>
@@ -137,14 +138,8 @@ struct cpu_common_dbs_info {
*/
struct mutex timer_mutex;

- /*
- * Per policy lock that serializes access to queuing work from timer
- * handlers.
- */
- spinlock_t timer_lock;
-
ktime_t time_stamp;
- unsigned int skip_work;
+ atomic_t skip_work;
struct work_struct work;
};