linux-kernel - Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush after TLB batching faiure

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Date:	Mon, 3 Nov 2014 17:56:33 +0000
From:	Will Deacon <will.deacon@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush
 after TLB batching faiure

On Sat, Nov 01, 2014 at 05:01:30PM +0000, Linus Torvalds wrote:
> On Wed, Oct 29, 2014 at 2:27 PM, Benjamin Herrenschmidt
> <benh@...nel.crashing.org> wrote:
> >
> > TLB flushing is only me I think, I'll engage my brain after breakfast
> > and see if is all good
> 
> Ping? Breakfast is either long over, of you're starting to look a bit
> like Mr Creosote...

Wafer thin patch?

> Anyway, Will, I assume this is not a correctness issue for you, just
> an annoying performance issue. Right? Or is there actually some issue
> with the actual range not being set to be sufficiently large?

Yeah, it's just a performance issue. For ranges over 1k pages, we end up
flushing the whole TLB.

> Also, it strikes me that I *think* that you might be able to extend
> your patch to remove the whole "need_flush" field, since as far as I
> can tell, "tlb->need_flush" is now equivalent to "tlb->start <
> tlb->end". Of course, as long as we still require that
> "need_flush_all", that doesn't actually save us any space, so maybe
> it's not worth changing.

We use `tlb->end > 0' in the arm64 backend, so I think you're right. I'll
take a look in the name of cleanup and this can wait until 3.19.

Will
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