Messages in this thread | | | Date | Fri, 20 Jan 2012 09:46:33 +0100 | From | Daniel Lezcano <> | Subject | Re: [PATCH 0/3] coupled cpuidle state support |
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On 12/21/2011 01:09 AM, Colin Cross wrote: > On some ARM SMP SoCs (OMAP4460, Tegra 2, and probably more), the > cpus cannot be independently powered down, either due to > sequencing restrictions (on Tegra 2, cpu 0 must be the last to > power down), or due to HW bugs (on OMAP4460, a cpu powering up > will corrupt the gic state unless the other cpu runs a work > around). Each cpu has a power state that it can enter without > coordinating with the other cpu (usually Wait For Interrupt, or > WFI), and one or more "coupled" power states that affect blocks > shared between the cpus (L2 cache, interrupt controller, and > sometimes the whole SoC). Entering a coupled power state must > be tightly controlled on both cpus. > > The easiest solution to implementing coupled cpu power states is > to hotplug all but one cpu whenever possible, usually using a > cpufreq governor that looks at cpu load to determine when to > enable the secondary cpus. This causes problems, as hotplug is an > expensive operation, so the number of hotplug transitions must be > minimized, leading to very slow response to loads, often on the > order of seconds. > > This patch series implements an alternative solution, where each > cpu will wait in the WFI state until all cpus are ready to enter > a coupled state, at which point the coupled state function will > be called on all cpus at approximately the same time. > > Once all cpus are ready to enter idle, they are woken by an smp > cross call. At this point, there is a chance that one of the > cpus will find work to do, and choose not to enter suspend. A > final pass is needed to guarantee that all cpus will call the > power state enter function at the same time. During this pass, > each cpu will increment the ready counter, and continue once the > ready counter matches the number of online coupled cpus. If any > cpu exits idle, the other cpus will decrement their counter and > retry. > > To use coupled cpuidle states, a cpuidle driver must: > > Set struct cpuidle_device.coupled_cpus to the mask of all > coupled cpus, usually the same as cpu_possible_mask if all cpus > are part of the same cluster. The coupled_cpus mask must be > set in the struct cpuidle_device for each cpu. > > Set struct cpuidle_device.safe_state to a state that is not a > coupled state. This is usually WFI. > > Set CPUIDLE_FLAG_COUPLED in struct cpuidle_state.flags for each > state that affects multiple cpus. > > Provide a struct cpuidle_state.enter function for each state > that affects multiple cpus. This function is guaranteed to be > called on all cpus at approximately the same time. The driver > should ensure that the cpus all abort together if any cpu tries > to abort once the function is called. > > This series was functionally tested on v3.0, but has only been > compile-tested on v3.2 after the removal of per-cpu state fields.
Hi Colin,
this patchset could be interesting to resolve in a generic way the cpu dependencies. What is the status of this patchset ?
Did you have the opportunity to measure the power consumption with and without this patchset ?
Thanks -- Daniel
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