[PATCH] ondemand governor : avoid too high frequency for stats

[PATCH] ondemand governor : avoid too high frequency for stats

[Eric Piel]

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Hello,

In the ondemand governor, there is a periodic measurement of the CPU 
usage. This CPU usage is updated by the scheduler after every tick 
(basically, by adding 1 either to "idle" or to "user" or to "system"). 
So if the frequency of the governor is too high, the stat will be 
meaningless (as mostly no number have changed).

So this patch checks that the measurements are separated by at least 10 
ticks. It means that by default, stats will have about 5% error (20 
ticks). Of course those numbers can be argued but, IMHO, they look sane. 
The patch also includes a small clean-up to check more explictly the 
result of the conversion from ns to µs being null.

Let's note that (on x86) this has never been really needed before 2.6.13 
because HZ was always 1000. Now that HZ can be 100, some CPU might be 
affected by this problem. For instance, the centrino which has a 10µs 
transition latency would lead to the governor allowing to read stats 
every tick (10ms)!

Please apply,
Eric Piel

--
From: Eric Piel <eric.piel@tremplin-utc.net>

Avoid the ondemand cpufreq goveror to use a too high frequency for stats.

Signed-off-by: Eric Piel <eric.piel@tremplin-utc.net>
--

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--- linux-2.6.13-rc6/drivers/cpufreq/cpufreq_ondemand.c.bak	2005-08-18 16:54:45.000000000 +0200
+++ linux-2.6.13-rc6/drivers/cpufreq/cpufreq_ondemand.c	2005-08-18 17:46:31.000000000 +0200
@@ -48,7 +48,10 @@
  * All times here are in uS.
  */
 static unsigned int 				def_sampling_rate;
-#define MIN_SAMPLING_RATE			(def_sampling_rate / 2)
+#define MIN_SAMPLING_RATE_RATIO			(2)
+/* for correct statistics, we need at least 10 ticks between each measure */
+#define MIN_STAT_SAMPLING_RATE			(MIN_SAMPLING_RATE_RATIO * jiffies_to_usecs(10))
+#define MIN_SAMPLING_RATE			(def_sampling_rate / MIN_SAMPLING_RATE_RATIO)
 #define MAX_SAMPLING_RATE			(500 * def_sampling_rate)
 #define DEF_SAMPLING_RATE_LATENCY_MULTIPLIER	(1000)
 #define DEF_SAMPLING_DOWN_FACTOR		(1)
@@ -416,13 +419,16 @@ static int cpufreq_governor_dbs(struct c
 		if (dbs_enable == 1) {
 			unsigned int latency;
 			/* policy latency is in nS. Convert it to uS first */
+			latency = policy->cpuinfo.transition_latency / 1000;
+			if (latency == 0)
+				latency = 1;
 
-			latency = policy->cpuinfo.transition_latency;
-			if (latency < 1000)
-				latency = 1000;
-
-			def_sampling_rate = (latency / 1000) *
+			def_sampling_rate = latency *
 					DEF_SAMPLING_RATE_LATENCY_MULTIPLIER;
+
+			if (def_sampling_rate < MIN_STAT_SAMPLING_RATE)
+				def_sampling_rate = MIN_STAT_SAMPLING_RATE;
+
 			dbs_tuners_ins.sampling_rate = def_sampling_rate;
 			dbs_tuners_ins.ignore_nice = 0;
 

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