[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>

1 files changed, 13 insertions, 1 deletions

diff --git a/Documentation/cpu-freq/governors.txt b/Documentation/cpu-freq/governors.txt
index 9b1851297d4..ce73f3eb5dd 100644
--- a/Documentation/cpu-freq/governors.txt
+++ b/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