[CPUFREQ] ondemand/conservative: sanitize sampling_rate restrictions

Limit sampling rate to transition_latency * 100 or kernel limits.
If sampling_rate is tried to be set too low, set the lowest allowed value.
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Dave Jones <davej@redhat.com>

Documentation/cpu-freq/governors.txt

@@ -117,7 +117,19 @@ accessible parameters:
 sampling_rate: measured in uS (10^-6 seconds), this is how often you
 want the kernel to look at the CPU usage and to make decisions on
 what to do about the frequency.  Typically this is set to values of
-around '10000' or more.
+around '10000' or more. It's default value is (cmp. with users-guide.txt):
+transition_latency * 1000
+The lowest value you can set is:
+transition_latency * 100 or it may get restricted to a value where it
+makes not sense for the kernel anymore to poll that often which depends
+on your HZ config variable (HZ=1000: max=20000us, HZ=250: max=5000).
+Be aware that transition latency is in ns and sampling_rate is in us, so you
+get the same sysfs value by default.
+Sampling rate should always get adjusted considering the transition latency
+To set the sampling rate 750 times as high as the transition latency
+in the bash (as said, 1000 is default), do:
+echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) \
+    >ondemand/sampling_rate
 
 show_sampling_rate_(min|max): THIS INTERFACE IS DEPRECATED, DON'T USE IT.
 You can use wider ranges now and the general

drivers/cpufreq/cpufreq_conservative.c

@@ -54,8 +54,20 @@ static unsigned int def_sampling_rate;
 			(MIN_SAMPLING_RATE_RATIO * jiffies_to_usecs(10))
 #define MIN_SAMPLING_RATE			\
 			(def_sampling_rate / MIN_SAMPLING_RATE_RATIO)
+/* Above MIN_SAMPLING_RATE will vanish with its sysfs file soon
+ * Define the minimal settable sampling rate to the greater of:
+ *   - "HW transition latency" * 100 (same as default sampling / 10)
+ *   - MIN_STAT_SAMPLING_RATE
+ * To avoid that userspace shoots itself.
+*/
+static unsigned int minimum_sampling_rate(void)
+{
+	return max(def_sampling_rate / 10, MIN_STAT_SAMPLING_RATE);
+}
+
+/* This will also vanish soon with removing sampling_rate_max */
 #define MAX_SAMPLING_RATE			(500 * def_sampling_rate)
-#define DEF_SAMPLING_RATE_LATENCY_MULTIPLIER	(1000)
+#define LATENCY_MULTIPLIER			(1000)
 #define DEF_SAMPLING_DOWN_FACTOR		(1)
 #define MAX_SAMPLING_DOWN_FACTOR		(10)
 #define TRANSITION_LATENCY_LIMIT		(10 * 1000 * 1000)
@@ -208,13 +220,11 @@ static ssize_t store_sampling_rate(struct cpufreq_policy *unused,
 	ret = sscanf(buf, "%u", &input);
 
 	mutex_lock(&dbs_mutex);
-	if (ret != 1 || input > MAX_SAMPLING_RATE ||
-	    input < MIN_SAMPLING_RATE) {
+	if (ret != 1) {
 		mutex_unlock(&dbs_mutex);
 		return -EINVAL;
 	}
-
-	dbs_tuners_ins.sampling_rate = input;
+	dbs_tuners_ins.sampling_rate = max(input, minimum_sampling_rate());
 	mutex_unlock(&dbs_mutex);
 
 	return count;
@@ -540,11 +550,9 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
 			if (latency == 0)
 				latency = 1;
 
-			def_sampling_rate = 10 * latency *
-					DEF_SAMPLING_RATE_LATENCY_MULTIPLIER;
-
-			if (def_sampling_rate < MIN_STAT_SAMPLING_RATE)
-				def_sampling_rate = MIN_STAT_SAMPLING_RATE;
+			def_sampling_rate =
+				max(10 * latency * LATENCY_MULTIPLIER,
+				    MIN_STAT_SAMPLING_RATE);
 
 			dbs_tuners_ins.sampling_rate = def_sampling_rate;
 

drivers/cpufreq/cpufreq_ondemand.c

@@ -52,8 +52,20 @@ static unsigned int def_sampling_rate;
 			(MIN_SAMPLING_RATE_RATIO * jiffies_to_usecs(10))
 #define MIN_SAMPLING_RATE			\
 			(def_sampling_rate / MIN_SAMPLING_RATE_RATIO)
+/* Above MIN_SAMPLING_RATE will vanish with its sysfs file soon
+ * Define the minimal settable sampling rate to the greater of:
+ *   - "HW transition latency" * 100 (same as default sampling / 10)
+ *   - MIN_STAT_SAMPLING_RATE
+ * To avoid that userspace shoots itself.
+*/
+static unsigned int minimum_sampling_rate(void)
+{
+	return max(def_sampling_rate / 10, MIN_STAT_SAMPLING_RATE);
+}
+
+/* This will also vanish soon with removing sampling_rate_max */
 #define MAX_SAMPLING_RATE			(500 * def_sampling_rate)
-#define DEF_SAMPLING_RATE_LATENCY_MULTIPLIER	(1000)
+#define LATENCY_MULTIPLIER			(1000)
 #define TRANSITION_LATENCY_LIMIT		(10 * 1000 * 1000)
 
 static void do_dbs_timer(struct work_struct *work);
@@ -255,13 +267,11 @@ static ssize_t store_sampling_rate(struct cpufreq_policy *unused,
 	ret = sscanf(buf, "%u", &input);
 
 	mutex_lock(&dbs_mutex);
-	if (ret != 1 || input > MAX_SAMPLING_RATE
-		     || input < MIN_SAMPLING_RATE) {
+	if (ret != 1) {
 		mutex_unlock(&dbs_mutex);
 		return -EINVAL;
 	}
-
-	dbs_tuners_ins.sampling_rate = input;
+	dbs_tuners_ins.sampling_rate = max(input, minimum_sampling_rate());
 	mutex_unlock(&dbs_mutex);
 
 	return count;
@@ -607,11 +617,9 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
 			if (latency == 0)
 				latency = 1;
 
-			def_sampling_rate = latency *
-					DEF_SAMPLING_RATE_LATENCY_MULTIPLIER;
-
-			if (def_sampling_rate < MIN_STAT_SAMPLING_RATE)
-				def_sampling_rate = MIN_STAT_SAMPLING_RATE;
+			def_sampling_rate =
+				max(latency * LATENCY_MULTIPLIER,
+				    MIN_STAT_SAMPLING_RATE);
 
 			dbs_tuners_ins.sampling_rate = def_sampling_rate;
 		}