On Thu, Oct 23, 2014 at 5:01 AM, Daniel Lezcano
<[email protected]> 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.
>
> 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 <[email protected]>
> ---
> 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;
> +
Unfortunately, you've touched ugly code,
and your (correct) patch makes it ever-so slightly more ugly,
instead of more clear.
I think the code would read more clearly, and your patch would
less obscure, if the code read something like this sow that it is
clear at the menu_select level when and where we monkey
with predicted_us:
menu_select()...
...
data->predicted_us = div_round64(bla bla bla
interactivity_overrride_us = get_typical_interval(data);
if (interactivity_override_us)
if (interactivity_predicted_us < data->predicted_us)
data->predicted_us = interactivity_override_us;
And, of course, down inside get_typical_interval()
...
if (!avg)
return 0;
...
if (likely(stddev <= ULONG_MAX)) {
...
return avg;
thanks,
Len Brown, Intel Open Source Technology Center
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