[PATCH 0/2] arm64: cpuidle: make arm_cpuidle_suspend() more efficient

[Jisheng Zhang]

Hi Lorenzo,

On Thu, 24 Mar 2016 16:44:19 +0000
Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> wrote:

> 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:

hmm, yes, it makes more sense to combined them into one patch.

> 
> Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>

Thanks for reviewing,
Jisheng