Query about: ARM11 MPCore: preemption/task migration cache coherency

[bill4carson@gmail.com (bill4carson)]

On 2012?05?31? 11:00, Catalin Marinas wrote:
> Hi,
>
> On Wed, May 30, 2012 at 11:01:59AM +0100, bill4carson wrote:
>> On 2012?05?30? 14:38, Will Deacon wrote:
>>> On Tue, May 29, 2012 at 06:28:11AM +0100, bill4carson wrote:
>>>> --- a/arch/arm/mm/cache-v6.S
>>>> +++ b/arch/arm/mm/cache-v6.S
>>>> @@ -170,6 +170,10 @@ ENDPROC(v6_coherent_kern_range)
>>>>     ENTRY(v6_flush_kern_dcache_area)
>>>>            add     r1, r0, r1
>>>>     1:
>>>> +#ifdef CONFIG_SMP
>>>> +       ldr     r2, [r0]                        @ read for ownership
>>>> +       str     r2, [r0]                        @ write for ownership
>>>> +#endif /* CONFIG_SMP */
>>>>     #ifdef HARVARD_CACHE
>>>>            mcr     p15, 0, r0, c7, c14, 1          @ clean&   invalidate D line
>>>>     #else
>>>
>>> I don't think the invalidation is needed here, so you probably don't need to
>>> hack this function at all.
> ...
>> 1. task t runs on CPU 0, it executes one program in nand flash,
>>      so first task t read *part* of this program into its local Dcache,
>>      let's call it part_a;
>>
>> 2. task t migrates from CPU0 onto CPU1, in there reads the rest of
>>      program into its local Dcache, label it part_b;
>>
>> 3. on CPU1, task t flush the whole address range of this program,
>>      As with ARM11 MPCore, this flush is locally effective, after flush
>>      part_a still resides in CPU0 L1 Dcache.
>>
>> 4. When this program gets executed, CPU 1 fetch its instructions from
>>      Icache, SCU will notice this action,
>>
>>      Question:
>>      At this point, is stale part_a in main memory *or* L1 Dcache part_a
>>      in CPU0 routed into CPU1 as instruction?
>
> This has been discussed in the past. The first thing is to disable
> CONFIG_PREEMPT to make sure you are not preempted between page copying
> and task execution. The code doing the page copy needs to call
> flush_dcache_page() on the CPU where the copy occurred and Linux does a
> non-lazy D-cache flush. But there are many situations where Linux
> doesn't do this (you can google for flush_dcache_page and my name to
> find a few places where I tried to fix this).
>
Thanks:)


> AFAIK, The SCU only snoops the D-cache, not the I-cache. We have a full
> I-cache invalidation during task migration.

                             ^^^^^^^^^^^^
Could you please point it to me?




-- 
Love each day!

--bill