linux-kernel - Re: [PATCH v1 3/4] cpufreq: Add special-purpose fast-switching callback for drivers

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Date:   Tue, 8 Dec 2020 14:32:07 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Doug Smythies <dsmythies@...us.net>,
        Giovanni Gherdovich <ggherdovich@...e.com>
Subject: Re: [PATCH v1 3/4] cpufreq: Add special-purpose fast-switching
 callback for drivers

On 07-12-20, 17:35, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> First off, some cpufreq drivers (eg. intel_pstate) can pass hints
> beyond the current target frequency to the hardware and there are no
> provisions for doing that in the cpufreq framework.  In particular,
> today the driver has to assume that it should not allow the frequency
> to fall below the one requested by the governor (or the required
> capacity may not be provided) which may not be the case and which may
> lead to excessive energy usage in some scenarios.
> 
> Second, the hints passed by these drivers to the hardware need not be
> in terms of the frequency, so representing the utilization numbers
> coming from the scheduler as frequency before passing them to those
> drivers is not really useful.
> 
> Address the two points above by adding a special-purpose replacement
> for the ->fast_switch callback, called ->adjust_perf, allowing the
> governor to pass abstract performance level (rather than frequency)
> values for the minimum (required) and target (desired) performance
> along with the CPU capacity to compare them to.
> 
> Also update the schedutil governor to use the new callback instead
> of ->fast_switch if present.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> Changes with respect to the RFC:
>  - Don't pass "busy" to ->adjust_perf().
>  - Use a special 'update_util' hook for the ->adjust_perf() case in
>    schedutil (this still requires an additional branch because of the
>    shared common code between this case and the "frequency" one, but
>    IMV this version is cleaner nevertheless).
> 
> ---
>  drivers/cpufreq/cpufreq.c        |   40 ++++++++++++++++++++++++++++++++
>  include/linux/cpufreq.h          |   14 +++++++++++
>  include/linux/sched/cpufreq.h    |    5 ++++
>  kernel/sched/cpufreq_schedutil.c |   48 +++++++++++++++++++++++++++++++--------
>  4 files changed, 98 insertions(+), 9 deletions(-)
> 
> Index: linux-pm/include/linux/cpufreq.h
> ===================================================================
> --- linux-pm.orig/include/linux/cpufreq.h
> +++ linux-pm/include/linux/cpufreq.h
> @@ -320,6 +320,15 @@ struct cpufreq_driver {
>  					unsigned int index);
>  	unsigned int	(*fast_switch)(struct cpufreq_policy *policy,
>  				       unsigned int target_freq);
> +	/*
> +	 * ->fast_switch() replacement for drivers that use an internal
> +	 * representation of performance levels and can pass hints other than
> +	 * the target performance level to the hardware.
> +	 */
> +	void		(*adjust_perf)(unsigned int cpu,
> +				       unsigned long min_perf,
> +				       unsigned long target_perf,
> +				       unsigned long capacity);

With this callback in place, do we still need to keep the other stuff we
introduced recently, like CPUFREQ_NEED_UPDATE_LIMITS ?

-- 
viresh