[PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record_transition()

[Viresh Kumar]

In order to prepare for lock-less stats update, add support to defer any
updates to it until cpufreq_stats_record_transition() is called.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 drivers/cpufreq/cpufreq_stats.c | 75 ++++++++++++++++++++++++---------
 1 file changed, 56 insertions(+), 19 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
index 94d959a8e954..3e7eee29ee86 100644
--- a/drivers/cpufreq/cpufreq_stats.c
+++ b/drivers/cpufreq/cpufreq_stats.c
@@ -22,17 +22,22 @@ struct cpufreq_stats {
 	spinlock_t lock;
 	unsigned int *freq_table;
 	unsigned int *trans_table;
+
+	/* Deferred reset */
+	unsigned int reset_pending;
+	unsigned long long reset_time;
 };
 
-static void cpufreq_stats_update(struct cpufreq_stats *stats)
+static void cpufreq_stats_update(struct cpufreq_stats *stats,
+				 unsigned long long time)
 {
 	unsigned long long cur_time = get_jiffies_64();
 
-	stats->time_in_state[stats->last_index] += cur_time - stats->last_time;
+	stats->time_in_state[stats->last_index] += cur_time - time;
 	stats->last_time = cur_time;
 }
 
-static void cpufreq_stats_clear_table(struct cpufreq_stats *stats)
+static void cpufreq_stats_reset_table(struct cpufreq_stats *stats)
 {
 	unsigned int count = stats->max_state;
 
@@ -41,42 +46,67 @@ static void cpufreq_stats_clear_table(struct cpufreq_stats *stats)
 	memset(stats->trans_table, 0, count * count * sizeof(int));
 	stats->last_time = get_jiffies_64();
 	stats->total_trans = 0;
+
+	/* Adjust for the time elapsed since reset was requested */
+	WRITE_ONCE(stats->reset_pending, 0);
+	cpufreq_stats_update(stats, stats->reset_time);
 	spin_unlock(&stats->lock);
 }
 
 static ssize_t show_total_trans(struct cpufreq_policy *policy, char *buf)
 {
-	return sprintf(buf, "%d\n", policy->stats->total_trans);
+	struct cpufreq_stats *stats = policy->stats;
+
+	if (READ_ONCE(stats->reset_pending))
+		return sprintf(buf, "%d\n", 0);
+	else
+		return sprintf(buf, "%d\n", stats->total_trans);
 }
 cpufreq_freq_attr_ro(total_trans);
 
 static ssize_t show_time_in_state(struct cpufreq_policy *policy, char *buf)
 {
 	struct cpufreq_stats *stats = policy->stats;
+	bool pending = READ_ONCE(stats->reset_pending);
+	unsigned long long time;
 	ssize_t len = 0;
 	int i;
 
 	if (policy->fast_switch_enabled)
 		return 0;
 
-	spin_lock(&stats->lock);
-	cpufreq_stats_update(stats);
-	spin_unlock(&stats->lock);
-
 	for (i = 0; i < stats->state_num; i++) {
+		if (pending) {
+			if (i == stats->last_index)
+				time = get_jiffies_64() - stats->reset_time;
+			else
+				time = 0;
+		} else {
+			time = stats->time_in_state[i];
+			if (i == stats->last_index)
+				time += get_jiffies_64() - stats->last_time;
+		}
+
 		len += sprintf(buf + len, "%u %llu\n", stats->freq_table[i],
-			(unsigned long long)
-			jiffies_64_to_clock_t(stats->time_in_state[i]));
+			       jiffies_64_to_clock_t(time));
 	}
 	return len;
 }
 cpufreq_freq_attr_ro(time_in_state);
 
+/* We don't care what is written to the attribute */
 static ssize_t store_reset(struct cpufreq_policy *policy, const char *buf,
 			   size_t count)
 {
-	/* We don't care what is written to the attribute. */
-	cpufreq_stats_clear_table(policy->stats);
+	struct cpufreq_stats *stats = policy->stats;
+
+	/*
+	 * Defer resetting of stats to cpufreq_stats_record_transition() to
+	 * avoid races.
+	 */
+	WRITE_ONCE(stats->reset_pending, 1);
+	stats->reset_time = get_jiffies_64();
+
 	return count;
 }
 cpufreq_freq_attr_wo(reset);
@@ -84,8 +114,9 @@ cpufreq_freq_attr_wo(reset);
 static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
 {
 	struct cpufreq_stats *stats = policy->stats;
+	bool pending = READ_ONCE(stats->reset_pending);
 	ssize_t len = 0;
-	int i, j;
+	int i, j, count;
 
 	if (policy->fast_switch_enabled)
 		return 0;
@@ -113,8 +144,13 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
 		for (j = 0; j < stats->state_num; j++) {
 			if (len >= PAGE_SIZE)
 				break;
-			len += scnprintf(buf + len, PAGE_SIZE - len, "%9u ",
-					stats->trans_table[i*stats->max_state+j]);
+
+			if (pending)
+				count = 0;
+			else
+				count = stats->trans_table[i * stats->max_state + j];
+
+			len += scnprintf(buf + len, PAGE_SIZE - len, "%9u ", count);
 		}
 		if (len >= PAGE_SIZE)
 			break;
@@ -228,10 +264,11 @@ void cpufreq_stats_record_transition(struct cpufreq_policy *policy,
 	struct cpufreq_stats *stats = policy->stats;
 	int old_index, new_index;
 
-	if (!stats) {
-		pr_debug("%s: No stats found\n", __func__);
+	if (!stats)
 		return;
-	}
+
+	if (READ_ONCE(stats->reset_pending))
+		cpufreq_stats_reset_table(stats);
 
 	old_index = stats->last_index;
 	new_index = freq_table_get_index(stats, new_freq);
@@ -241,7 +278,7 @@ void cpufreq_stats_record_transition(struct cpufreq_policy *policy,
 		return;
 
 	spin_lock(&stats->lock);
-	cpufreq_stats_update(stats);
+	cpufreq_stats_update(stats, stats->last_time);
 
 	stats->last_index = new_index;
 	stats->trans_table[old_index * stats->max_state + new_index]++;
-- 
2.25.0.rc1.19.g042ed3e048af