On Thu, 23 Oct 2014, Daniel Lezcano wrote:
> The first time the 'get_typical_function' is called, it computes an average
> of zero as no data is filled yet. That leads the 'data->predicted_us' variable
> to be set to zero too.
>
> The caller, 'menu_select' will then do:
>
> interactivity_req = data->predicted_us /
> performance_multiplier(nr_iowaiters, cpu_load);
>
> That sets the interactivity_req to zero (0/performance...).
>
> and then
>
> if (latency_req > interactivity_req)
> latency_req = interactivity_req;
>
> ... setting 'latency_req' to zero too.
>
> No idle state will fulfill this constraint and we will go the C1 state as
> default and leading to an update. So the next calls will compute an average
> different from zero.
>
> Even if that works with the current code but with a broken semantic, it will
> just break with the next patches where we are stricter with the latencies
> check: the first check will fail (latency_req is zero), then no update will
> occur leading to always falling to choose an idle state.
s/falling/failing/
>
> As there are no previous values and it is pointless to compute a standard
> deviation for these unexisting values. Just return without setting the
> 'data->predicted_us' to zero.
>
> Signed-off-by: Daniel Lezcano <daniel.lezc...@linaro.org>
Acked-by: Nicolas Pitre <n...@linaro.org>
> ---
> drivers/cpuidle/governors/menu.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/drivers/cpuidle/governors/menu.c
> b/drivers/cpuidle/governors/menu.c
> index 3907301..6ae8390 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -226,6 +226,15 @@ again:
> else
> do_div(avg, divisor);
>
> + /*
> + * We are at the very beginning and no data have been filled
> + * yet. Let's skip the standard deviation computation
> + * otherwise the data->predicted_us will be zero and that will
> + * lead to a zero latency req in the select function
> + */
> + if (!avg)
> + return;
> +
> /* Then try to determine standard deviation */
> stddev = 0;
> for (i = 0; i < INTERVALS; i++) {
> --
> 1.9.1
>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
