RE: [PATCH v2] ARM: perf: save/restore pmu registers in pm notifier

[Neil Zhang]

> -----Original Message-----
> From: Will Deacon [mailto:will.deacon@xxxxxxx]
> Sent: 2014年4月15日 20:50
> To: Neil Zhang
> Cc: linux@xxxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Sudeep Holla
> Subject: Re: [PATCH v2] ARM: perf: save/restore pmu registers in pm notifier
> 
> On Tue, Apr 15, 2014 at 01:46:08PM +0100, Neil Zhang wrote:
> > > On Tue, Apr 15, 2014 at 01:37:17PM +0100, Neil Zhang wrote:
> > > > > On Mon, Apr 14, 2014 at 02:42:22AM +0100, Neil Zhang wrote:
> > > > > > From: Sudeep KarkadaNagesha <sudeep.karkadanagesha@xxxxxxx>
> > > > > >
> > > > > > This adds core support for saving and restoring CPU PMU
> > > > > > registers for suspend/resume support i.e. deeper C-states in cpuidle
> terms.
> > > > > > This patch adds support only to ARMv7 PMU registers save/restore.
> > > > > > It needs to be extended to xscale and ARMv6 if needed.
> > > > > >
> > > > > > [Neil] We found that DS-5 not work on our CA7 based SoCs.
> > > > > > After debuging, found PMU registers were lost because of core
> > > > > > power
> > > down.
> > > > > > Then i found Sudeep had a patch to fix it about two years ago
> > > > > > but not in the mainline, just port it.
> > > > >
> > > > > What I don't like about this patch is that we're introducing
> > > > > significant overhead for SoCs that don't require save/restore of
> > > > > the PMU state. I'd much rather see core power down disabled
> > > > > whilst the PMU is in use but, if that's not possible, then I think we need
> to:
> > > > >
> > > > >  (1) Make this conditional for cores that really need it
> > > > >
> > > > >  (2) Only save/restore if the PMU is in use (even better, just
> save/restore
> > > > >      the live registers, but that's probably not worth the effort
> > > > >      initially).
> > > > >
> > > >
> > > > The patch has check the ARMV7_PMNC_E bit when save / restore, so
> > > > suppose only the core's that use PMU will do the save / restore work.
> > >
> > > Seems pretty fragile to base our save/restore decision on the value
> > > of one of the registers, though. What happens if the control
> > > register is zeroed by the core power down?
> > >
> > It will check the saved control value when restore, so is should be OK
> > while control register is zeroed.
> 
> Ah yes, I mixed up and save and restore functions. It's still horrible that we
> have to read the control register unconditionally during the save though
> -- it might be nicer if we simply register/unregister the notifier during perf runs
> (in the same way that we request/free the PMU IRQs).
> 

Thanks for the comments, I will refine it according to your suggestion.

> Will

Best Regards,
Neil Zhang