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

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Date:	Fri, 22 Feb 2013 15:25:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.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 Fri, 2013-02-22 at 15:16 +0100, Ingo Molnar wrote:
> > I checked arch/x86/include/asm/atomic64_32.h and we use 
> > cmpxchg8b for everything from _set() to _read(), which 
> > translates into 'horridly stupendifyingly slow' for a number 
> > of machines, but coherent.
> 
> That's a valid concern - and cmpxchg8b is the only 64-bit op 
> available on most 32-bit x86 CPUs which does not involve the 
> FPU.
> 
> Wondering how significant this range of x86 problem boxes will 
> be by the time any of these changes reaches upstream and distros 
> - and how much 'horridly stupendifyingly slow' is in terms of 
> cycles expended.

On the !x86 side of things we're implementing (generic) atomic64 using
hashed spinlocks, so there too using a single spinlock around the
entire data structure is a complete win.

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