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>

2 files changed, 38 insertions, 16 deletions

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index c78af11a51f0..e5a08a13ca84 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -304,6 +304,7 @@ static void gov_cancel_work(struct cpufreq_policy *policy)
         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 @@ static void dbs_work_handler(struct work_struct *work)
         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 @@ static void dbs_update_util_handler(struct update_util_data *data, u64 time,
 {
         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 @@ static void dbs_update_util_handler(struct update_util_data *data, u64 time,
          * - 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;
-
-                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 (policy_dbs->work_in_progress)
+                return;
+
+        /*
+         * 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 @@ static int cpufreq_governor_start(struct cpufreq_policy *policy)
         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 8138eff5e25b..521daac38ba5 100644
--- a/drivers/cpufreq/cpufreq_governor.h
+++ b/drivers/cpufreq/cpufreq_governor.h
@@ -130,6 +130,9 @@ struct policy_dbs_info {
         /* 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,