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From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-arm-kernel@lists.infradead.org, Ard.Biesheuvel@arm.com
Subject: Re: [PATCH v1 0/5] arm64: avoid out-of-line ll/sc atomics
Date: Fri, 17 May 2019 09:24:01 +0200
Message-ID: <20190517072401.GI2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190516155344.24060-1-andrew.murray@arm.com>

On Thu, May 16, 2019 at 04:53:39PM +0100, Andrew Murray wrote:
> When building for LSE atomics (CONFIG_ARM64_LSE_ATOMICS), if the hardware
> or toolchain doesn't support it the existing code will fallback to ll/sc
> atomics. It achieves this by branching from inline assembly to a function
> that is built with specical compile flags. Further this results in the
> clobbering of registers even when the fallback isn't used increasing
> register pressure.
> 
> Let's improve this by providing inline implementatins of both LSE and
> ll/sc and use a static key to select between them. This allows for the
> compiler to generate better atomics code.

Don't you guys have alternatives? That would avoid having both versions
in the code, and thus significantly cuts back on the bloat.

> These changes add a small amount of bloat on defconfig according to
> bloat-o-meter:
> 
> text:
>   add/remove: 1/108 grow/shrink: 3448/20 up/down: 272768/-4320 (268448)
>   Total: Before=12363112, After=12631560, chg +2.17%

I'd say 2% is quite significant bloat.

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  parent reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16 15:53 Andrew Murray
2019-05-16 15:53 ` [PATCH v1 1/5] jump_label: Don't warn on __exit jump entries Andrew Murray
2019-05-16 15:53 ` [PATCH v1 2/5] arm64: Use correct ll/sc atomic constraints Andrew Murray
2019-05-16 15:53 ` [PATCH v1 3/5] arm64: atomics: avoid out-of-line ll/sc atomics Andrew Murray
2019-05-16 15:53 ` [PATCH v1 4/5] arm64: avoid using hard-coded registers for LSE atomics Andrew Murray
2019-05-16 15:53 ` [PATCH v1 5/5] arm64: atomics: remove atomic_ll_sc compilation unit Andrew Murray
2019-05-17  7:24 ` Peter Zijlstra [this message]
2019-05-17 10:08   ` [PATCH v1 0/5] arm64: avoid out-of-line ll/sc atomics Andrew Murray
2019-05-17 10:29     ` Ard Biesheuvel
2019-05-22 10:45       ` Andrew Murray
2019-05-22 11:44         ` Ard Biesheuvel
2019-05-22 15:36           ` Andrew Murray
2019-05-17 12:05     ` Peter Zijlstra
2019-05-17 12:19       ` Ard Biesheuvel

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