Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the flushing algorithm

[Borislav Petkov <bp@alien8.de>]

On Thu, Jun 22, 2017 at 08:55:36AM -0700, Andy Lutomirski wrote:
> > Ah, simple: we control the flushing with info.new_tlb_gen and
> > mm->context.tlb_gen. I.e., this check:
> >
> >
> >         if (f->end != TLB_FLUSH_ALL &&
> >             f->new_tlb_gen == local_tlb_gen + 1 &&
> >             f->new_tlb_gen == mm_tlb_gen) {
> >
> > why can't we write:
> >
> >         if (f->end != TLB_FLUSH_ALL &&
> >             mm_tlb_gen == local_tlb_gen + 1)
> >
> > ?
> 
> Ah, I thought you were asking about why I needed mm_tlb_gen ==
> local_tlb_gen + 1.  This is just an optimization, or at least I hope
> it is.  The idea is that, if we know that another flush is coming, it
> seems likely that it would be faster to do a full flush and increase
> local_tlb_gen all the way to mm_tlb_gen rather than doing a partial
> flush, increasing local_tlb_gen to something less than mm_tlb_gen, and
> needing to flush again very soon.

Thus the f->new_tlb_gen check whether it is local_tlb_gen + 1.

Btw, do you see how confusing this check is: you have new_tlb_gen from
a variable passed from the function call IPI, local_tlb_gen which is
the CPU's own and then there's also mm_tlb_gen which we've written into
new_tlb_gen from:

	info.new_tlb_gen = bump_mm_tlb_gen(mm);

which incremented mm_tlb_gen too.

> Hmm.  I'd be nervous that there are more subtle races if we do this.
> For example, suppose that a partial flush increments tlb_gen from 1 to
> 2 and a full flush increments tlb_gen from 2 to 3.  Meanwhile, the CPU
> is busy switching back and forth between mms, so the partial flush
> sees the cpu set in mm_cpumask but the full flush doesn't see the cpu
> set in mm_cpumask.

Lemme see if I understand this correctly: you mean, the full flush will
exit early due to the

	if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(loaded_mm))) {

test?

> The flush IPI hits after a switch_mm_irqs_off() call notices the
> change from 1 to 2. switch_mm_irqs_off() will do a full flush and
> increment the local tlb_gen to 2, and the IPI handler for the partial
> flush will see local_tlb_gen == mm_tlb_gen - 1 (because local_tlb_gen
> == 2 and mm_tlb_gen == 3) and do a partial flush.

Why, the 2->3 flush has f->end == TLB_FLUSH_ALL.

That's why you have this thing in addition to the tlb_gen.

What we end up doing in this case, is promote the partial flush to a
full one and thus have a partial and a full flush which are close by
converted to two full flushes.

> The problem here is that it's not obvious to me that this actually
> ends up flushing everything that's needed. Maybe all the memory
> ordering gets this right, but I can imagine scenarios in which
> switch_mm_irqs_off() does its flush early enough that the TLB picks up
> an entry that was supposed to get zapped by the full flush.

See above.

And I don't think that having two full flushes back-to-back is going to
cost a lot as the second one won't flush a whole lot.

> IOW it *might* be valid, but I think it would need very careful review
> and documentation.

Always.

Btw, I get the sense this TLB flush avoiding scheme becomes pretty
complex for diminishing reasons.

  [ Or maybe I'm not seeing them - I'm always open to corrections. ]

Especially if intermediary levels from the pagetable walker are cached
and reestablishing the TLB entries seldom means a full walk.

You should do a full fledged benchmark to see whether this whole
complexity is even worth it, methinks.

-- 
Regards/Gruss,
    Boris.

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