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

Viresh Kumar
2020-12-15T04:17:48Z

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@intel.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@intel.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 ?

Ping

--
viresh