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

Viresh Kumar
2020-12-08T09:02:58Z

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 ?

--
viresh