linux-kernel - Re: [RFC/RFT][PATCH v3 2/2] cpufreq: schedutil: Switching frequencies from interrupt context

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Message-ID: <20160225091249.GV6375@twins.programming.kicks-ass.net>
Date:	Thu, 25 Feb 2016 10:12:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC/RFT][PATCH v3 2/2] cpufreq: schedutil: Switching
 frequencies from interrupt context

On Thu, Feb 25, 2016 at 10:08:40AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 25, 2016 at 12:30:43AM +0100, Rafael J. Wysocki wrote:
> > +unsigned int acpi_cpufreq_fast_switch(struct cpufreq_policy *policy,
> > +				      unsigned int target_freq)
> > +{
> > +	struct acpi_cpufreq_data *data = policy->driver_data;
> > +	struct cpufreq_frequency_table *entry;
> > +	struct acpi_processor_performance *perf;
> > +	unsigned int uninitialized_var(next_perf_state);
> > +	unsigned int uninitialized_var(next_freq);
> > +	unsigned int best_diff;
> > +
> > +	for (entry = data->freq_table, best_diff = UINT_MAX;
> > +	     entry->frequency != CPUFREQ_TABLE_END; entry++) {
> > +		unsigned int diff, freq = entry->frequency;
> > +
> > +		if (freq == CPUFREQ_ENTRY_INVALID)
> > +			continue;
> > +
> > +		diff = abs(freq - target_freq);
> 
> Why would you consider frequencies that are below where you want to be?

Also, if you look for the first largest freq (which would make most
sense I think) and this table is sorted, you can do a binary search.

Then again, not sure the table is big enough to make that worth it.

> > +		if (diff >= best_diff)
> > +			continue;
> > +
> > +		best_diff = diff;
> > +		next_perf_state = entry->driver_data;
> > +		next_freq = freq;
> > +		if (best_diff == 0)
> > +			goto found;
> > +	}
> > +	if (best_diff == UINT_MAX)
> > +		return CPUFREQ_ENTRY_INVALID;
> > +
> > + found:
> > +	perf = to_perf_data(data);
> > +	if (perf->state == next_perf_state) {
> > +		if (unlikely(data->resume))
> > +			data->resume = 0;
> > +		else
> > +			return next_freq;
> > +	}
> > +
> > +	data->cpu_freq_fast_write(perf->states[next_perf_state].control);
> > +	perf->state = next_perf_state;
> > +	return next_freq;
> > +}