Messages in this thread |  | | Date | Mon, 27 Oct 2014 22:48:57 -0400 | Subject | Re: [PATCH V2 4/5] cpuidle: menu: Fix the get_typical_interval | From | Len Brown <> |
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On Thu, Oct 23, 2014 at 5:01 AM, Daniel Lezcano <daniel.lezcano@linaro.org> 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 <daniel.lezcano@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; > +
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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