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authorDave Jones <davej@redhat.com>2005-09-20 12:39:35 -0700
committerDave Jones <davej@redhat.com>2005-09-20 12:39:35 -0700
commitdf8b59be0976c56820453730078bef99a8d1dbda (patch)
treea836bfc1588a20ca997b878bac82188240a471af
parentc90eef84b8f5c7416cb99ab6907896f2dd392b4d (diff)
[CPUFREQ] Avoid the ondemand cpufreq governor to use a too high frequency for stats.
The problem is 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 when HZ=100, the centrino ,which has a 10µs transition latency, would lead to the governor allowing to read stats every tick (10ms)! Signed-off-by: Eric Piel <eric.piel@tremplin-utc.net> Signed-off-by: Dave Jones <davej@redhat.com>
-rw-r--r--drivers/cpufreq/cpufreq_ondemand.c18
1 files changed, 12 insertions, 6 deletions
diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
index c1fc9c62bb5..17741111246 100644
--- a/drivers/cpufreq/cpufreq_ondemand.c
+++ b/drivers/cpufreq/cpufreq_ondemand.c
@@ -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 cpufreq_policy *policy,
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;