[CPUFREQ] add sampling_down_factor tunable to improve ondemand performance

Adds a new global tunable, sampling_down_factor.  Set to 1 it makes no
changes from existing behavior, but set to greater than 1 (e.g. 100)
it acts as a multiplier for the scheduling interval for reevaluating
load when the CPU is at its top speed due to high load.  This improves
performance by reducing the overhead of load evaluation and helping
the CPU stay at its top speed when truly busy, rather than shifting
back and forth in speed.  This tunable has no effect on behavior at
lower speeds/lower CPU loads.

This patch is against 2.6.36-rc6.

This patch should help solve kernel bug 19672 "ondemand is slow".
Signed-off-by: NDavid Niemi <dniemi@verisign.com>
Acked-by: NVenkatesh Pallipadi <venki@google.com>
CC: Daniel Hollocher <danielhollocher@gmail.com>
CC: <cpufreq-list@vger.kernel.org>
CC: <linux-kernel@vger.kernel.org>
Signed-off-by: NDave Jones <davej@redhat.com>

drivers/cpufreq/cpufreq_ondemand.c

@@ -30,6 +30,8 @@
 
 #define DEF_FREQUENCY_DOWN_DIFFERENTIAL		(10)
 #define DEF_FREQUENCY_UP_THRESHOLD		(80)
+#define DEF_SAMPLING_DOWN_FACTOR		(1)
+#define MAX_SAMPLING_DOWN_FACTOR		(100000)
 #define MICRO_FREQUENCY_DOWN_DIFFERENTIAL	(3)
 #define MICRO_FREQUENCY_UP_THRESHOLD		(95)
 #define MICRO_FREQUENCY_MIN_SAMPLE_RATE		(10000)
@@ -82,6 +84,7 @@ struct cpu_dbs_info_s {
 	unsigned int freq_lo;
 	unsigned int freq_lo_jiffies;
 	unsigned int freq_hi_jiffies;
+	unsigned int rate_mult;
 	int cpu;
 	unsigned int sample_type:1;
 	/*
@@ -108,10 +111,12 @@ static struct dbs_tuners {
 	unsigned int up_threshold;
 	unsigned int down_differential;
 	unsigned int ignore_nice;
+	unsigned int sampling_down_factor;
 	unsigned int powersave_bias;
 	unsigned int io_is_busy;
 } dbs_tuners_ins = {
 	.up_threshold = DEF_FREQUENCY_UP_THRESHOLD,
+	.sampling_down_factor = DEF_SAMPLING_DOWN_FACTOR,
 	.down_differential = DEF_FREQUENCY_DOWN_DIFFERENTIAL,
 	.ignore_nice = 0,
 	.powersave_bias = 0,
@@ -259,6 +264,7 @@ static ssize_t show_##file_name						\
 show_one(sampling_rate, sampling_rate);
 show_one(io_is_busy, io_is_busy);
 show_one(up_threshold, up_threshold);
+show_one(sampling_down_factor, sampling_down_factor);
 show_one(ignore_nice_load, ignore_nice);
 show_one(powersave_bias, powersave_bias);
 
@@ -340,6 +346,29 @@ static ssize_t store_up_threshold(struct kobject *a, struct attribute *b,
 	return count;
 }
 
+static ssize_t store_sampling_down_factor(struct kobject *a,
+			struct attribute *b, const char *buf, size_t count)
+{
+	unsigned int input, j;
+	int ret;
+	ret = sscanf(buf, "%u", &input);
+
+	if (ret != 1 || input > MAX_SAMPLING_DOWN_FACTOR || input < 1)
+		return -EINVAL;
+	mutex_lock(&dbs_mutex);
+	dbs_tuners_ins.sampling_down_factor = input;
+
+	/* Reset down sampling multiplier in case it was active */
+	for_each_online_cpu(j) {
+		struct cpu_dbs_info_s *dbs_info;
+		dbs_info = &per_cpu(od_cpu_dbs_info, j);
+		dbs_info->rate_mult = 1;
+	}
+	mutex_unlock(&dbs_mutex);
+
+	return count;
+}
+
 static ssize_t store_ignore_nice_load(struct kobject *a, struct attribute *b,
 				      const char *buf, size_t count)
 {
@@ -401,6 +430,7 @@ static ssize_t store_powersave_bias(struct kobject *a, struct attribute *b,
 define_one_global_rw(sampling_rate);
 define_one_global_rw(io_is_busy);
 define_one_global_rw(up_threshold);
+define_one_global_rw(sampling_down_factor);
 define_one_global_rw(ignore_nice_load);
 define_one_global_rw(powersave_bias);
 
@@ -409,6 +439,7 @@ static struct attribute *dbs_attributes[] = {
 	&sampling_rate_min.attr,
 	&sampling_rate.attr,
 	&up_threshold.attr,
+	&sampling_down_factor.attr,
 	&ignore_nice_load.attr,
 	&powersave_bias.attr,
 	&io_is_busy.attr,
@@ -562,6 +593,10 @@ static void dbs_check_cpu(struct cpu_dbs_info_s *this_dbs_info)
 
 	/* Check for frequency increase */
 	if (max_load_freq > dbs_tuners_ins.up_threshold * policy->cur) {
+		/* If switching to max speed, apply sampling_down_factor */
+		if (policy->cur < policy->max)
+			this_dbs_info->rate_mult =
+				dbs_tuners_ins.sampling_down_factor;
 		dbs_freq_increase(policy, policy->max);
 		return;
 	}
@@ -584,6 +619,9 @@ static void dbs_check_cpu(struct cpu_dbs_info_s *this_dbs_info)
 				(dbs_tuners_ins.up_threshold -
 				 dbs_tuners_ins.down_differential);
 
+		/* No longer fully busy, reset rate_mult */
+		this_dbs_info->rate_mult = 1;
+
 		if (freq_next < policy->min)
 			freq_next = policy->min;
 
@@ -607,7 +645,8 @@ static void do_dbs_timer(struct work_struct *work)
 	int sample_type = dbs_info->sample_type;
 
 	/* We want all CPUs to do sampling nearly on same jiffy */
-	int delay = usecs_to_jiffies(dbs_tuners_ins.sampling_rate);
+	int delay = usecs_to_jiffies(dbs_tuners_ins.sampling_rate
+		* dbs_info->rate_mult);
 
 	if (num_online_cpus() > 1)
 		delay -= jiffies % delay;
@@ -711,6 +750,7 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
 			}
 		}
 		this_dbs_info->cpu = cpu;
+		this_dbs_info->rate_mult = 1;
 		ondemand_powersave_bias_init_cpu(cpu);
 		/*
 		 * Start the timerschedule work, when this governor