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

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Message-ID: <20201215041636.yfgyswqjslg4hlff@vireshk-i7>
Date:   Tue, 15 Dec 2020 09:46:36 +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 08-12-20, 14:32, Viresh Kumar wrote:
> 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 ?

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-- 
viresh