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

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Date:	Fri, 22 Feb 2013 15:18:10 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.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


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:
> > > Argh!! at what cost? 64bit atomics are like expensive. Wouldn't
> > adding
> > > a seqlock be saner?
> > 
> > Not sure. This requires a spinlock in the write side which is called
> > from
> > fast path like the timer interrupt.
> 
> A single spinlock is _way_ cheaper than a ton of cmpxchg8b()s 
> to update a few variables.

Every cmpxchg8b() will be roughly as expensive as a spinlock 
acquire+release fastpath.

Thanks,

	Ingo
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