cpufreq: governor: Avoid atomic operations in hot paths
Rework the handling of work items by dbs_update_util_handler() and
dbs_work_handler() so the former (which is executed in scheduler
paths) only uses atomic operations when absolutely necessary. That
is, when the policy is shared and dbs_update_util_handler() has
already decided that this is the time to queue up a work item.
In particular, this avoids the atomic ops entirely on platforms where
policy objects are never shared.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index c78af11..e5a08a1 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -304,6 +304,7 @@
irq_work_sync(&policy_dbs->irq_work);
cancel_work_sync(&policy_dbs->work);
atomic_set(&policy_dbs->work_count, 0);
+ policy_dbs->work_in_progress = false;
}
static void dbs_work_handler(struct work_struct *work)
@@ -326,13 +327,15 @@
policy_dbs->sample_delay_ns = jiffies_to_nsecs(delay);
mutex_unlock(&policy_dbs->timer_mutex);
+ /* Allow the utilization update handler to queue up more work. */
+ atomic_set(&policy_dbs->work_count, 0);
/*
- * If the atomic operation below is reordered with respect to the
- * sample delay modification, the utilization update handler may end
- * up using a stale sample delay value.
+ * If the update below is reordered with respect to the sample delay
+ * modification, the utilization update handler may end up using a stale
+ * sample delay value.
*/
- smp_mb__before_atomic();
- atomic_dec(&policy_dbs->work_count);
+ smp_wmb();
+ policy_dbs->work_in_progress = false;
}
static void dbs_irq_work(struct irq_work *irq_work)
@@ -348,6 +351,7 @@
{
struct cpu_dbs_info *cdbs = container_of(data, struct cpu_dbs_info, update_util);
struct policy_dbs_info *policy_dbs = cdbs->policy_dbs;
+ u64 delta_ns;
/*
* The work may not be allowed to be queued up right now.
@@ -355,17 +359,30 @@
* - Work has already been queued up or is in progress.
* - It is too early (too little time from the previous sample).
*/
- if (atomic_inc_return(&policy_dbs->work_count) == 1) {
- u64 delta_ns;
+ if (policy_dbs->work_in_progress)
+ return;
- delta_ns = time - policy_dbs->last_sample_time;
- if ((s64)delta_ns >= policy_dbs->sample_delay_ns) {
- policy_dbs->last_sample_time = time;
- irq_work_queue(&policy_dbs->irq_work);
- return;
- }
- }
- atomic_dec(&policy_dbs->work_count);
+ /*
+ * If the reads below are reordered before the check above, the value
+ * of sample_delay_ns used in the computation may be stale.
+ */
+ smp_rmb();
+ delta_ns = time - policy_dbs->last_sample_time;
+ if ((s64)delta_ns < policy_dbs->sample_delay_ns)
+ return;
+
+ /*
+ * If the policy is not shared, the irq_work may be queued up right away
+ * at this point. Otherwise, we need to ensure that only one of the
+ * CPUs sharing the policy will do that.
+ */
+ if (policy_dbs->is_shared &&
+ !atomic_add_unless(&policy_dbs->work_count, 1, 1))
+ return;
+
+ policy_dbs->last_sample_time = time;
+ policy_dbs->work_in_progress = true;
+ irq_work_queue(&policy_dbs->irq_work);
}
static struct policy_dbs_info *alloc_policy_dbs_info(struct cpufreq_policy *policy,
@@ -542,6 +559,8 @@
if (!policy->cur)
return -EINVAL;
+ policy_dbs->is_shared = policy_is_shared(policy);
+
sampling_rate = dbs_data->sampling_rate;
ignore_nice = dbs_data->ignore_nice_load;
diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
index 8138eff..521daac 100644
--- a/drivers/cpufreq/cpufreq_governor.h
+++ b/drivers/cpufreq/cpufreq_governor.h
@@ -130,6 +130,9 @@
/* dbs_data may be shared between multiple policy objects */
struct dbs_data *dbs_data;
struct list_head list;
+ /* Status indicators */
+ bool is_shared; /* This object is used by multiple CPUs */
+ bool work_in_progress; /* Work is being queued up or in progress */
};
static inline void gov_update_sample_delay(struct policy_dbs_info *policy_dbs,