linux-kernel - Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

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Date:	Fri, 10 Dec 2010 15:45:15 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Mikael Pettersson <mikpe@...uu.se>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM

On Fri, 10 Dec 2010, Eric Dumazet wrote:

> > There is certainly a major difference in that execution of a stream of
> > instructions on the same cpu is guaranteed to have a coherent view of
> > the data. That is not affected by interrupts etc.
> >
>
> We dont care of interrupts. We care of doing a transaction over a
> complex set of data, that cannot be done using an atomic op (or we need
> a spinlock/mutex/rwlock), and should not because of performance.

So what? The cpu gives you incoherent view of data somewhere when only
processing data from a single cpu?

If you have remote data accesses (loop summing the data?) and you have to
be concerned about data coherence then you CANNOT use this_cpu_ops since
they are not guaranteed to be coherent to other processors.
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