On 23/11/15 14:28, Juri Lelli wrote: > Hi all, > Hi again everybody, thanks a lot to Rob and Vincent for feedback, but, IMHO, we'd need more discussion happening on this series to figure out how we move forward; so, this is a ping :-). It's a simple information that we need to figure out how to provide, and in which form, but it has the potential to form a standardized basis for achieving important performance benefits. Thanks, - Juri > ARM systems may be configured to have CPUs with different power/performance > characteristics within the same chip. In this case, additional information has > to be made available to the kernel (the scheduler in particular) for it to be > aware of such differences and take decisions accordingly. This posting stems > from the ongoing discussion about introducing a simple platform energy cost > model to guide scheduling decisions (a.k.a Energy Aware Scheduling [1]), but > also aims to be an independent track aimed to standardise the way we make the > scheduler aware of heterogenous CPU systems. With these patches and in addition > patches from [1] (that make the scheduler wakeup paths aware of heterogenous > CPU systems) we enable the scheduler to have good default performance on such > systems. In addition, we get a clearly defined way of providing the scheduler > with needed information about CPU capacity on such systems. > > CPU capacity is defined in this context as a number that provides the scheduler > information about CPUs heterogeneity. Such heterogeneity can come from > micro-architectural differences (e.g., ARM big.LITTLE systems) or maximum > frequency at which CPUs can run (e.g., SMP systems with multiple frequency > domains and different max frequencies). Heterogeneity in this context is about > differing performance characteristics; in practice, the binding that we propose > in this RFC tries to capture a first-order approximation of the relative > performance of CPUs. > > This RFC proposes a solution to the problem of how do we init CPUs original > capacity. The way it works today, and for arm A15/A7 systems only, is that we > rely on cpu_efficiency magic numbers from arch/arm/kernel/topology.c and the > existence of clock-frequency dtb properties; having those values available, we > then do some math to come up with capacities we know from measurement (e.g., > EAS energy model), e.g. for TC2 they are 430 for A7 and 1024 for A15. > Currently, arm64 doesn't have such a feature at all. > > With this patchset we provide CPUs capacity information either from DT or from > sysfs interface. Such information is standardized for both arm and arm64. > > Patches high level description: > > o 01/08 cleans up how cpu_scale is initialized in arm > o 02/08 introduces documentation for the new optional DT binding > o [03-06]/08 add cpu-capacity attribute to TC2 and Juno DTs and provide > parsing of such information at boot time > o [07-08]/08 introduce sysfs attribute > > The patchset is based on top of tip/sched/core as of today. > > In case you would like to test this out, I pushed a branch here: > > git://linux-arm.org/linux-jl.git upstream/default_caps_dt > > This branch contains additional patches, useful to better understand how CPU > capacity information is actually used by the scheduler. Discussion regarding > these additional patches will be started with a different posting in the > future. We just didn't want to make discussion too broad, as we realize that > this set can be controversial already on its own. > > Comments, concerns and rants are more than welcome! > > Best, > > - Juri > > Juri Lelli (8): > ARM: initialize cpu_scale to its default > Documentation: arm: define DT cpu capacity bindings > arm: parse cpu capacity from DT > arm, dts: add TC2 cpu capacity information > arm64: parse cpu capacity from DT > arm64, dts: add Juno cpu capacity information > arm: add sysfs cpu_capacity attribute > arm64: add sysfs cpu_capacity attribute > > .../devicetree/bindings/arm/cpu-capacity.txt | 227 +++++++++++++++++++++ > Documentation/devicetree/bindings/arm/cpus.txt | 17 ++ > arch/arm/boot/dts/vexpress-v2p-ca15_a7.dts | 6 + > arch/arm/kernel/topology.c | 122 ++++++++++- > arch/arm64/boot/dts/arm/juno.dts | 7 + > arch/arm64/kernel/topology.c | 114 +++++++++++ > 6 files changed, 489 insertions(+), 4 deletions(-) > create mode 100644 Documentation/devicetree/bindings/arm/cpu-capacity.txt > > -- > 2.2.2 > -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
