linux-kernel - Re: [RFC][PATCH 2/5] mm: Switch mod_state() to __this_cpu

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Date:	Tue, 20 Sep 2011 11:07:27 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [RFC][PATCH 2/5] mm: Switch mod_state() to __this_cpu_read()

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.

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