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

[heechul Yun]

On Wed, Jun 1, 2011 at 5:55 AM, Lorenzo Pieralisi
<lorenzo.pieralisi at arm.com> wrote:
> 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?

The kernel version is 2.6.36.
I checked the number of IPIs before/after the program and found there
were close to none IPIs during the run.

>>
>> 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.
>

You mean cache/tlb operations can automatically generate maintenance
request to other cores (not via IPIs) ?

> Lorenzo
>
>
>