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Date: Tue, 20 Sep 2011 11:07:27 -0500 (CDT)
From: Christoph Lameter <cl@gentwo.org>
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To: Steven Rostedt <rostedt@goodmis.org>
cc: Thomas Gleixner <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Christoph Lameter <cl@linux.com>, Tejun Heo <htejun@gmail.com>
Subject: Re: [RFC][PATCH 2/5] mm: Switch mod_state() to __this_cpu_read()
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On Tue, 20 Sep 2011, Steven Rostedt wrote:

> > NO!! This defeats the whole purpose of this_cpu_ops and make the whole
> > scheme utterly useless.
>
> The thing is, the whole purpose was broken to begin with. Defeating a
> broken design is a good thing!

So then we are not allowed to use segment prefixes in core code to avoid
preempt enable disable? And to avoid interrupt disabling enabling in
critical allocator sections?

> > There are trivial cases like counter increments that are not a problem at
> > all. Most use cases are those. More complex ones can be developed to avoid
> > various overhead in performance critical sections of the kernel.
> >
>
> And adding a preempt_disable; this_cpu_inc(); preempt_enable; is not a
> bad thing either.

It defeats the purpose of the whole thing.

> What benchmarks do you have that shows this helped in anything????

The various patchsets that went into the kernel had benchmarks results.