A page fault causes exiting from WFI mode for all idling cores?

[Lorenzo Pieralisi]

On Wed, 2011-06-01 at 11:14 +0100, Catalin Marinas wrote:
> On 28 May 2011 12:59, heechul Yun <heechul at illinois.edu> wrote:
> > While I was investigating page fault handling performance on a quad
> > core Cortex-A9 ARM processor, I noticed that whenever there is a
> > page fault, all idling cores also awakened---exiting from WFI instruction
> > on the idle loop (and go back to WFI again).
> >
> > Since the page fault generating program is running on a single core,
> > it seems unnecessary to wake up other idling cores (hence saving
> > power) as well.
> >
> > Can anyone tell me if this, exiting from WFI on every page fault, is
> > correct behavior? If so, why?
> 
> I don't think this is explicitly done by Linux, at least not with
> Cortex-A9. For ARM11MPCore, we do some TLB operations which are
> broadcast in software via IPI. Which kernel version are you using?
> 
> Also note that WFI is just a hint to the processor, so it could get
> out of it at any time. Maybe the A9 just does this when it gets some
> TLB ops from another CPU.
> 

According to A9 TRM transition from standby to run mode can be triggered
by cp15 maintenance requests generated by another cpu, just to reiterate
Catalin's reply.

Lorenzo