linux-kernel - Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb

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Date:   Fri, 23 Jun 2017 10:42:19 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the
 flushing algorithm

On Thu, Jun 22, 2017 at 11:08:38AM -0700, Andy Lutomirski wrote:
> Yes, I agree it's confusing.  There really are three numbers.  Those
> numbers are: the latest generation, the generation that this CPU has
> caught up to, and the generation that the requester of the flush we're
> currently handling has asked us to catch up to.  I don't see a way to
> reduce the complexity.

Yeah, can you pls put that clarification what what is, over it. It
explains it nicely what the check is supposed to do.

> >> 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.
> 
> Yes.  The idea is that we only do remote partial flushes when it's
> 100% obvious that it's safe.

So why wouldn't my simplified suggestion work then?

	if (f->end != TLB_FLUSH_ALL &&
	     mm_tlb_gen == local_tlb_gen + 1)

1->2 is a partial flush - gets promoted to a full one
2->3 is a full flush - it will get executed as one due to the f->end setting to
TLB_FLUSH_ALL.

> It could be converted to two full flushes or to just one, I think,
> depending on what order everything happens in.

Right. One flush at the right time would be optimal.

> But this approach of using three separate tlb_gen values seems to
> cover all the bases, and I don't think it's *that* bad.

Sure.

As I said in IRC, let's document that complexity then so that when we
stumble over it in the future, we at least know why it was done this
way.

-- 
Regards/Gruss,
    Boris.

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