Invoking the cpufreq driver to set a frequency can be expensive. On platforms with a cpufreq driver that does not support fast switching a thread must be woken to complete the operation. IPIs will also occur if and when support to process remote task wakeups is added in schedutil.
Currently schedutil calculates a raw frequency request from scheduler utilization data. This raw frequency request does not correlate to supported cpufreq driver frequencies aside from being capped by the CPU's maximum frequency. Consequently, there may be many consecutive requests for different raw frequency values which all translate to the same driver-supported frequency. For example on a platform with 3 supported frequencies 200MHz, 400MHz, and 600MHz, raw requests for 257MHz, 389MHz, and 307MHz all map to a driver-supported frequency of 400MHz in schedutil. Assuming these requests were consecutive and there were no changes in policy limits (min/max), there is no need to issue the second or third request. In order to resolve a raw frequency request to a driver-supported one a new cpufreq API is added, cpufreq_driver_resolve_freq(). This API relies on a new cpufreq driver callback in the case of ->target() style drivers. Otherwise it uses the existing frequency table operations. Lookups are cached both in cpufreq_driver_resolve_freq() (for the benefit of the driver) and in schedutil. Changes since v2: - incorporated feedback from Viresh to use resolve_freq driver callbacks only for ->target() style drivers, to use cpufreq's freq table operations, and move freq mapping caching into cpufreq policy Changes since v1: - incorporated feedback from Rafael to avoid referencing freq_table from schedutil by introducing a new cpufreq API Steve Muckle (3): cpufreq: add cpufreq_driver_resolve_freq() cpufreq: schedutil: map raw required frequency to driver frequency cpufreq: acpi-cpufreq: use cached frequency mapping when possible drivers/cpufreq/acpi-cpufreq.c | 5 ++++- drivers/cpufreq/cpufreq.c | 25 +++++++++++++++++++++++++ include/linux/cpufreq.h | 16 ++++++++++++++++ kernel/sched/cpufreq_schedutil.c | 31 +++++++++++++++++++++++-------- 4 files changed, 68 insertions(+), 9 deletions(-) -- 2.7.3
