• Rafael J. Wysocki's avatar
    intel_pstate: Avoid extra invocation of intel_pstate_sample() · febce40f
    Rafael J. Wysocki authored
    The initialization of intel_pstate for a given CPU involves populating
    the fields of its struct cpudata that represent the previous sample,
    but currently that is done in a problematic way.
    
    Namely, intel_pstate_init_cpu() makes an extra call to
    intel_pstate_sample() so it reads the current register values that
    will be used to populate the "previous sample" record during the
    next invocation of intel_pstate_sample().  However, after commit
    a4675fbc (cpufreq: intel_pstate: Replace timers with utilization
    update callbacks) that doesn't work for last_sample_time, because
    the time value is passed to intel_pstate_sample() as an argument now.
    Passing 0 to it from intel_pstate_init_cpu() is problematic, because
    that causes cpu->last_sample_time == 0 to be visible in
    get_target_pstate_use_performance() (and hence the extra
    cpu->last_sample_time > 0 check in there) and effectively allows
    the first invocation of intel_pstate_sample() from
    intel_pstate_update_util() to happen immediately after the
    initialization which may lead to a significant "turn on"
    effect in the governor algorithm.
    
    To mitigate that issue, rework the initialization to avoid the
    extra intel_pstate_sample() call from intel_pstate_init_cpu().
    Instead, make intel_pstate_sample() return false if it has been
    called with cpu->sample.time equal to zero, which will make
    intel_pstate_update_util() skip the sample in that case, and
    reset cpu->sample.time from intel_pstate_set_update_util_hook()
    to make the algorithm start properly every time the hook is set.
    Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
    febce40f
intel_pstate.c 35.9 KB