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Date: Fri, 22 Feb 2013 15:23:05 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Kevin Hilman <khilman@linaro.org>,
        Russell King <rmk+kernel@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linaro-kernel@lists.linaro.org
Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats
Message-ID: <20130222142302.GC18149@somewhere.redhat.com>
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On Fri, Feb 22, 2013 at 06:21:31AM -0800, Eric Dumazet wrote:
> On Fri, 2013-02-22 at 15:09 +0100, Peter Zijlstra 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.
> 
> We also have include/linux/u64_stats_sync.h since 2.6.36

Interesting, we should probably use that instead.