Re: TLB and PTE coherency during munmap

[Catalin Marinas <catalin.marinas@arm.com>]

On 3 June 2013 10:16, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Fri, May 31, 2013 at 08:09:17AM +0400, Max Filippov wrote:
>> > Hi Peter,
>> >
>> > On Wed, May 29, 2013 at 9:51 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> > > What about something like this?
>> >
>> > With that patch I still get mtest05 firing my TLB/PTE incoherency
>> > check in the UP PREEMPT_VOLUNTARY configuration. This happens after
>> > zap_pte_range completion in the end of unmap_region because of
>> > rescheduling called in the following call chain:
>>
>> OK, so there two options; completely kill off fast-mode or something
>> like the below where we add magic to the scheduler :/
>>
>> I'm aware people might object to something like the below -- but since
>> its a possibility I thought we ought to at least mention it.
>>
>> For those new to the thread; the problem is that since the introduction
>> of preemptible mmu_gather the traditional UP fast-mode is broken.
>> Fast-mode is where we free the pages first and flush TLBs later. This is
>> not a problem if there's no concurrency, but obviously if you can
>> preempt there now is.
>>
>> I think I prefer completely killing off fast-mode esp. since UP seems to
>> go the way of the Dodo and it does away with an exception in the
>> mmu_gather code.
>>
>> Anyway; opinions? Linus, Thomas, Ingo?
>
> Since UP kernels have not been packaged up by major distros for years, and
> since the live-patching of SMP kernels (the SMP alternative-instructions
> patching machinery) does away with a big chunk of the SMP cost, I guess UP
> kernels are slowly becoming like TINY_RCU: interesting but not really a
> primary design goal?
>
> ( Another reason for reducing SMP vs. UP complexity in this area would be
>   the fact that we had a few bad regressions lately - the TLB code is not
>   getting simpler, and bugs are getting discovered and fixed slower. )
>
> At least that's the x86 perspective. ARM might still see it differently?

On ARM there is a lot of ongoing work on single zImage for multiple
SoCs and this implies SMP kernels. There is an SMP_ON_UP feature which
does run-time code patching to optimise the UP case in a few places.

Regarding tlb_fast_mode(), the ARM-specific implementation is always 0
on ARMv7 even if UP because of speculative TLB loads (the MMU could
pretty much act as a separate processor).

Catalin