linux-kernel - Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

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Date:	Fri, 22 Feb 2013 09:46:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

On Thu, 2013-02-21 at 21:56 -0800, Kevin Hilman wrote:
> On 64-bit platforms, reads/writes of the various cpustat fields are
> atomic due to native 64-bit loads/stores.  However, on non 64-bit
> platforms, reads/writes of the cpustat fields are not atomic and could
> lead to inconsistent statistics.

Which is a problem how?

> This problem was originally reported by Frederic Weisbecker as a
> 64-bit limitation with the nsec granularity cputime accounting for
> full dynticks, but then we realized that it's a problem that's been
> around for awhile and not specific to the new cputime accounting.
> 
> This series fixes this by first converting all access to the cputime
> fields to use accessor functions, and then converting the accessor
> functions to use the atomic64 functions.

Argh!! at what cost? 64bit atomics are like expensive. Wouldn't adding
a seqlock be saner?

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