linux-kernel - Re: [PATCH] arm64/lib: copy

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Date: Mon, 24 Jun 2024 18:56:33 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/lib: copy_page: s/stnp/stp

On Thu, Jun 13, 2024 at 08:18:12AM +0800, Jisheng Zhang wrote:
> stnp performs non-temporal store, give a hints to the memory system
> that caching is not useful for this data. But the scenario where
> copy_page() used may not have this implication, although I must admit
> there's such case where stnp helps performance(good). In this good
> case, we can rely on the HW write streaming mechanism in some
> implementations such as cortex-a55 to detect the case and take actions.
> 
> testing with https://github.com/apinski-cavium/copy_page_benchmark
> this patch can reduce the time by about 3% on cortex-a55 platforms.

What about other CPUs? I'm also not convinced by such microbenchmarks.
It looks like it always copies to the same page, the stp may even
benefit from some caching of the data which we wouldn't need in a real
scenario.

So, I'm not merging this unless it's backed by some solid data across
several CPU implementations.

-- 
Catalin