linux-kernel - Re: [RESEND RFC PATCH v1 5/5] arm64/mm: Elide tlbi in contpte

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Message-ID: <20241219163736.GG24724@willie-the-truck>
Date: Thu, 19 Dec 2024 16:37:37 +0000
From: Will Deacon <will@...nel.org>
To: Mikołaj Lenczewski <miko.lenczewski@....com>
Cc: ryan.roberts@....com, catalin.marinas@....com, corbet@....net,
	maz@...nel.org, oliver.upton@...ux.dev, joey.gouly@....com,
	suzuki.poulose@....com, yuzenghui@...wei.com,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev
Subject: Re: [RESEND RFC PATCH v1 5/5] arm64/mm: Elide tlbi in
 contpte_convert() under BBML2

On Wed, Dec 11, 2024 at 04:01:41PM +0000, Mikołaj Lenczewski wrote:
> If we support BBM level 2, we can potentially avoid an intermediate
> TLB invalidation, as hardware is capable of managing the TLB itself
> in this situation. Hardware will either silently clear out the
> offending entry, or will take a TLB Conflict Abort Exception.
> 
> Note that such aborts should not occur on Arm hardware and indeed
> were not seen on any of the benchmarked systems.
> 
> Eliding the invalidation results in a 12% improvement on a
> microbenchmark which targeted the worst case of contpte_convert(), which
> represents an 80% reduction in the overhead of contpte_convert().

Can you run something more indicative of real world performance than a
targetted microbenchmark please?

Will