• Dave Jones's avatar
    [CPUFREQ] Avoid the ondemand cpufreq governor to use a too high frequency for stats. · df8b59be
    Dave Jones authored
    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: default avatarEric Piel <eric.piel@tremplin-utc.net>
    Signed-off-by: default avatarDave Jones <davej@redhat.com>
    df8b59be
cpufreq_ondemand.c 13 KB