linux-kernel - Re: [PATCH V2 4/5] cpuidle: menu: Fix the get_typical

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Date:	Thu, 23 Oct 2014 12:43:58 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
cc:	rjw@...ysocki.net, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org
Subject: Re: [PATCH V2 4/5] cpuidle: menu: Fix the get_typical_interval

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.lezcano@...aro.org>

Acked-by: Nicolas Pitre <nico@...aro.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
> 
> 
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