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

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Date: Wed, 26 Jun 2024 19:08:42 +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 Wed, Jun 26, 2024 at 07:50:57PM +0800, Jisheng Zhang wrote:
> On Mon, Jun 24, 2024 at 06:56:33PM +0100, Catalin Marinas wrote:
> > 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.
[...]
> > 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.
> 
> Yep this is also my understanding where's the improvement from. And
> I must admit there's case where stnp helps performance. we can rely
> on the HW write streaming mechanism to detect and take actions.

Well, is that case realistic? Can you show any improvement with some
real-world uses? Most likely modern CPUs fall back to non-temporal
stores after a series of STPs but it depends on how soon they do it, how
much cache gets polluted. OTOH, page copying could be the result of a
CoW and we'd expect subsequent accesses from the user where some caching
may be beneficial.

So, hard to tell but we should make a decision based on a microbenchmark
that writes over the same page multiple times. If you have some
real-world tests that exercise this path (e.g. CoW, Android app startup)
and show an improvement, I'd be in favour of this. Otherwise, no.

Thanks.

-- 
Catalin