On Thu, Mar 24, 2016 at 09:18:53PM +0800, Jisheng Zhang wrote: > Hi Will, > > On Thu, 24 Mar 2016 11:15:07 +0000 Will Deacon wrote: > > > On Thu, Mar 24, 2016 at 01:08:48PM +0800, Jisheng Zhang wrote: > > > This series is to improve the arm_cpuidle_suspend() a bit by > > > removing/moving > > > out checks from this hot path. > > > > > > Jisheng Zhang (2): > > > arm64: cpuidle: remove cpu_ops check from arm_cpuidle_suspend() > > > arm64: cpuidle: make arm_cpuidle_suspend() a bit more efficient > > > > > > arch/arm64/kernel/cpuidle.c | 9 ++------- > > > 1 file changed, 2 insertions(+), 7 deletions(-) > > > > These look fine to me, but do you have any rough numbers showing what > > sort of improvement we get from this change? > > Good question. Here it is: > > I measured the 4096 * time from arm_cpuidle_suspend entry point to the > cpu_psci_cpu_suspend entry point. HW platform is Marvell BG4CT STB board. > > 1. only one shell, no other process, hot-unplug secondary cpus, execute the > following cmd > > while true > do > sleep 0.2 > done > > before the patch: 1581220ns > > after the patch: 1579630ns > > reduced by 0.1% > > 2. only one shell, no other process, hot-unplug secondary cpus, execute the > following cmd > > while true > do > md5sum /tmp/testfile > sleep 0.2 > done > > NOTE the testfile size should be larger than L1+L2 cache size > > before the patch: 1961960ns > after the patch: 1912500ns > > reduced by 2.5% > > So the more complex the system load, the bigger the improvement.
So between arm_cpuidle_suspend() and psci_cpu_suspend_enter() the checks that you are removing are almost the *only* code that is currently executed and this patch saves us best case 12ns per idle state entry (which is noise compared to CPU PM notifiers/FW execution time) if I am not mistaken, I can't wait to use that energy for something more useful :) Anyway, as a clean-up your patches are fine it is sloppy to check those pointers on every idle state entry (do you really need two patches ?), so: Acked-by: Lorenzo Pieralisi <lorenzo.pieral...@arm.com>
