Hi all, this is take 4 of "CPUs capacity information for heterogeneous systems" patchset [1]; some context follows. 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 RFC stems from the ongoing discussion about introducing a simple platform energy cost model to guide scheduling decisions (a.k.a Energy Aware Scheduling [2]), but also aims to be an independent track aimed to standardise the way we make the scheduler aware of heterogeneous CPU systems. With these patches and in addition patches from [2] (that make the scheduler wakeup paths aware of heterogeneous 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. After discussing at the recent Linaro Connect pros and cons of the alternatives presented on the list, we seem to come to the conclusion that a new DT binding is reasonable: the new property is only a first-order approximation that it is useful to get acceptable good behaviours during boot and early execution; it can be then overwritten using the sysfs interface if needed. I thus rebased v1 of this set on mainline as of today and I also removed the capacity-scale property (as agreed during v1 review): CPU capacity properties are now normalized w.r.t. the biggest capacity found while parsing the DT. The capacity property name and definition didn't change w.r.t. v1, as I intended this to be an almost pure refresh of that posting. Comments and feedback on what needs to be changed there is highly welcome. Patches high level description: o 01/08 cleans up how cpu_scale is initialized in arm (already landed on Russell's patch system) 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 mainline as of today (4.5). Changelogs comments regarding changes relative to previous versions, where present, refer to v1, as v2 and v3 didn't contain 02-06. In case you would like to test this out, I pushed a branch here: git://linux-arm.org/linux-jl.git upstream/default_caps_v4 This branch contains additional patches, useful to better understand how CPU capacity information is actually used by the scheduler. However, discussion regarding these additional patches is outside the scope of this posting. Best, - Juri [1] v1 - https://lkml.org/lkml/2015/11/23/391 v2 - https://lkml.org/lkml/2016/1/8/417 v3 - https://lkml.org/lkml/2016/2/3/405 [2] https://lkml.org/lkml/2015/7/7/754 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 | 222 +++++++++++++++++++++ Documentation/devicetree/bindings/arm/cpus.txt | 9 + arch/arm/boot/dts/vexpress-v2p-ca15_a7.dts | 5 + arch/arm/kernel/topology.c | 150 +++++++++++++- arch/arm64/boot/dts/arm/juno.dts | 6 + arch/arm64/kernel/topology.c | 143 +++++++++++++ 6 files changed, 531 insertions(+), 4 deletions(-) create mode 100644 Documentation/devicetree/bindings/arm/cpu-capacity.txt -- 2.7.0 -- 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
