Re: TLB and PTE coherency during munmap

[Ingo Molnar <mingo@kernel.org>]

* 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?

Thanks,

	Ingo