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Date:   Wed, 1 Jul 2020 16:16:19 +0530
From:   Viresh Kumar <viresh.kumar@linaro.org>
To:     Ionela Voinescu <ionela.voinescu@arm.com>
Cc:     rjw@rjwysocki.net, catalin.marinas@arm.com, sudeep.holla@arm.com,
        will@kernel.org, linux@armlinux.org.uk, valentin.schneider@arm.com,
        mingo@redhat.com, peterz@infradead.org, dietmar.eggemann@arm.com,
        linux-pm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/8] cpufreq: move invariance setter calls in cpufreq core
Message-ID: <20200701095219.gxrkowtukosnfmwp@vireshk-i7>
References: <20200701090751.7543-1-ionela.voinescu@arm.com>
 <20200701090751.7543-3-ionela.voinescu@arm.com>
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On 01-07-20, 10:07, Ionela Voinescu wrote:
> From: Valentin Schneider <valentin.schneider@arm.com>
> 
> To properly scale its per-entity load-tracking signals, the task scheduler
> needs to be given a frequency scale factor, i.e. some image of the current
> frequency the CPU is running at. Currently, this scale can be computed
> either by using counters (APERF/MPERF on x86, AMU on arm64), or by
> piggy-backing on the frequency selection done by cpufreq.
> 
> For the latter, drivers have to explicitly set the scale factor
> themselves, despite it being purely boiler-plate code: the required
> information depends entirely on the kind of frequency switch callback
> implemented by the driver, i.e. either of: target_index(), target(),
> fast_switch() and setpolicy().
> 
> The fitness of those callbacks with regard to driving the Frequency
> Invariance Engine (FIE) is studied below:
> 
> target_index()
> ==============
> Documentation states that the chosen frequency "must be determined by
> freq_table[index].frequency". It isn't clear if it *has* to be that
> frequency, or if it can use that frequency value to do some computation
> that ultimately leads to a different frequency selection. All drivers
> go for the former, while the vexpress-spc-cpufreq has an atypical
> implementation.
> 
> Thefore, the hook works on the asusmption the core can use
> freq_table[index].frequency.
> 
> target()
> =======
> This has been flagged as deprecated since:
> 
>   commit 9c0ebcf78fde ("cpufreq: Implement light weight ->target_index() routine")
> 
> It also doesn't have that many users:
> 
>   cpufreq-nforce2.c:371:2:	.target = nforce2_target,
>   cppc_cpufreq.c:416:2:		.target = cppc_cpufreq_set_target,
>   pcc-cpufreq.c:573:2:		.target = pcc_cpufreq_target,
> 
> Should we care about drivers using this hook, we may be able to exploit
> cpufreq_freq_transition_{being, end}(). Otherwise, if FIE support is
> desired in their current state, arch_set_freq_scale() could still be
> called directly by the driver, while CPUFREQ_CUSTOM_SET_FREQ_SCALE
> could be used to mark support for it.
> 
> fast_switch()
> =============
> This callback *has* to return the frequency that was selected.
> 
> setpolicy()
> ===========
> This callback does not have any designated way of informing what was the
> end choice. But there are only two drivers using setpolicy(), and none
> of them have current FIE support:
> 
>   drivers/cpufreq/longrun.c:281:	.setpolicy	= longrun_set_policy,
>   drivers/cpufreq/intel_pstate.c:2215:	.setpolicy	= intel_pstate_set_policy,
> 
> The intel_pstate is known to use counter-driven frequency invariance.

Same for acpi-cpufreq driver as well ?

And I think we should do the freq-invariance thing for all the above categories
nevertheless.

> If FIE support is desired in their current state, arch_set_freq_scale()
> could still be called directly by the driver, while
> CPUFREQ_CUSTOM_SET_FREQ_SCALE could be used to mark support for it.
> 
> Conclusion
> ==========
> 
> Given that the significant majority of current FIE enabled drivers use
> callbacks that lend themselves to triggering the setting of the FIE scale
> factor in a generic way, move the invariance setter calls to cpufreq core,
> while filtering drivers that flag custom support using
> CPUFREQ_CUSTOM_SET_FREQ_SCALE.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/cpufreq/cpufreq.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 0128de3603df..83b58483a39b 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2046,9 +2046,16 @@ EXPORT_SYMBOL(cpufreq_unregister_notifier);
>  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>  					unsigned int target_freq)
>  {
> +	unsigned int freq;
> +
>  	target_freq = clamp_val(target_freq, policy->min, policy->max);
> +	freq = cpufreq_driver->fast_switch(policy, target_freq);
> +

> +	if (freq && !(cpufreq_driver->flags & CPUFREQ_CUSTOM_SET_FREQ_SCALE))
> +		arch_set_freq_scale(policy->related_cpus, freq,
> +				    policy->cpuinfo.max_freq);

This needs to be a separate function.

>  
> -	return cpufreq_driver->fast_switch(policy, target_freq);
> +	return freq;
>  }
>  EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
>  
> @@ -2140,7 +2147,7 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
>  			    unsigned int relation)
>  {
>  	unsigned int old_target_freq = target_freq;
> -	int index;
> +	int index, retval;
>  
>  	if (cpufreq_disabled())
>  		return -ENODEV;
> @@ -2171,7 +2178,14 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
>  
>  	index = cpufreq_frequency_table_target(policy, target_freq, relation);
>  
> -	return __target_index(policy, index);
> +	retval = __target_index(policy, index);
> +
> +	if (!retval && !(cpufreq_driver->flags & CPUFREQ_CUSTOM_SET_FREQ_SCALE))
> +		arch_set_freq_scale(policy->related_cpus,
> +				    policy->freq_table[index].frequency,

policy->cur gets updated for both target and target_index type drivers. You can
use that safely. It gets updated after the postchange notification.

> +				    policy->cpuinfo.max_freq);
> +
> +	return retval;
>  }
>  EXPORT_SYMBOL_GPL(__cpufreq_driver_target);

-- 
viresh