linux-kernel - Re: [PATCH] cpufreq: stats: Do not populate stats when policy->cur has no exact match

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Message-ID: <1423628.uM8DtnYzDe@vostro.rjw.lan>
Date:	Sat, 16 Nov 2013 02:10 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Nishanth Menon <nm@...com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>, cpufreq@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, ceh@...com,
	Tobias Diedrich <ranma+xen@...edrich.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dave Jones <davej@...hat.com>
Subject: Re: [PATCH] cpufreq: stats: Do not populate stats when policy->cur has no exact match

On Friday, November 15, 2013 06:20:43 PM Nishanth Menon wrote:
> commit 46a310b ([CPUFREQ] Don't set stat->last_index to -1 if the
> pol->cur has incorrect value.) tries to handle case where policy->cur
> does not match any entry in freq_table.
> 
> As indicated in the above commit, the exact match search of
> freq_table_get index will return a -1 which is stored in
> stat->last_index. However, as a result of the above commit,
> cpufreq_stat_notifier_trans which updates the statistics, fails
> to update any *further* valid transitions that take place as
> stat->last_index is -1 as the condition occurs at boot and never
> solved.
> 
> So, instead of having a statistics information that never ever
> reflects valid data in the mentioned case and scratching our heads, we
> instead, refuse to populate any of the statistics entries and note in
> kernel log the error condition for developers to fix. The only useable
> information are the available frequencies which is already available
> in other cpufreq sysfs entries.
> 
> Cc: Tobias Diedrich <ranma+xen@...edrich.de>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Dave Jones <davej@...hat.com>
> Reported-by: Carlos Hernandez <ceh@...com>
> Signed-off-by: Nishanth Menon <nm@...com>

I like this one.  Any objections from anyone?

> ---
> 
> Patch based on v3.12 tag
> 
> Reported by Carlos on TI vendor kernel (v3.12tag based):
> 
> OMAP5uEVM: http://pastebin.mozilla.org/3612196
> AM335x-evm: http://pastebin.mozilla.org/3612220
> 
> As can be seen, the translation table are present, however none of the
> transition information is ever updated.
> With the current patch, this becomes: http://pastebin.mozilla.org/3612256
> 
> Original commit thread seems to be:
> https://lkml.org/lkml/2011/6/16/760 but I dont see much information if
> this was discussed further.
> 
> An alternate approach possible is to try and recover from this case by
> using an 'atmost' match to find a closest match and then giveup when
> we cant find one, but that does not really indicate a proper statistic
> (if freq1 < cur_freq < freq2, was the intent to be at freq1 or freq2?
> stats should not make that policy decision)
> 	http://pastebin.mozilla.org/3612179 (alternate approach 1)
> 
> Yet another option might be to update last_index using the first
> transition into a valid index, but then, we wont have statistics
> information 100% correct as we did not store the very first frequency
> stat.
> 	 http://pastebin.mozilla.org/3612241 (alternate approach 2)
> 
>  drivers/cpufreq/cpufreq_stats.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> index 4cf0d28..4c8a501 100644
> --- a/drivers/cpufreq/cpufreq_stats.c
> +++ b/drivers/cpufreq/cpufreq_stats.c
> @@ -251,6 +251,14 @@ static int cpufreq_stats_create_table(struct cpufreq_policy *policy,
>  	stat->last_time = get_jiffies_64();
>  	stat->last_index = freq_table_get_index(stat, policy->cur);
>  	spin_unlock(&cpufreq_stats_lock);
> +
> +	if (stat->last_index == -1) {
> +		pr_err("%s: No match for current freq %u in table. Disabled!\n",
> +		       __func__, policy->cur);
> +		ret = -EINVAL;
> +		goto error_out;
> +	}
> +
>  	cpufreq_cpu_put(current_policy);
>  	return 0;
>  error_out:
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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