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

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Message-ID: <CA+55aFwNALj9=R-V7cMK3_dT8TP_aGjFQiswSk8yegD_dzj9Wg@mail.gmail.com>
Date:	Tue, 28 Oct 2014 11:03:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush
 after TLB batching faiure

On Tue, Oct 28, 2014 at 10:07 AM, Will Deacon <will.deacon@....com> wrote:
>
> I don't think that's necessarily true, at least not on the systems I'm
> familiar with. A table walk can be comparatively expensive, particularly
> when virtualisation is involved and the depth of the host and guest page
> tables starts to grow -- we're talking >20 memory accesses per walk. By
> contrast, the TLB invalidation messages are asynchronous and carried on
> the interconnect (a DSB instruction is used to synchronise the updates).

">20 memory accesses per *walk*"? Isn't the ARM a regular table? So
once you've gone down to the pte level, it's just an array, regardless
of how many levels there are.

But I guess there are no actual multi-socket ARM's around in real
life, so you probably don't see the real scaling costs. Within a die,
you're probably right that the overhead is negligible.

                   Linus
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