linux-kernel - Re: [RFC/PATCH 2/5] kernel_cpustat: convert to atomic 64-bit accessors

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Message-ID: <CAFTL4hzJ8YXfCaHBaHZ5X8jRJA+zaC0FEaP-=-+5b=SrpXPLiw@mail.gmail.com>
Date:	Thu, 21 Feb 2013 23:15:50 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Kevin Hilman <khilman@...aro.org>,
	Mats Liljegren <mats.liljegren@...a.com>,
	linaro-dev@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC/PATCH 2/5] kernel_cpustat: convert to atomic 64-bit accessors

2013/2/21 Russell King - ARM Linux <linux@....linux.org.uk>:
> On Thu, Feb 21, 2013 at 10:53:07PM +0100, Frederic Weisbecker wrote:
>> That too should be kcpustat_this_cpu_set(), or kcpustat_this_cpu_add()
>> FWIW. But we probably don't need the overhead of atomic_add() that
>> does a LOCK.
>> atomic_set(var, atomic_read(var) + delta) would be better. All we need
>
> You mean atomic64_set() and atomic64_read().

Right.

> Looking at the generic
> version in lib/atomic64.c, atomic64_add() is cheaper for 32-bit arches
> because it doesn't involve taking the lock twice.

Yeah but that's the generic version. I guess the archs that have
overriden don't take any locks there.
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