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Date: Thu, 10 Oct 2013 15:34:34 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
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On Thu, 10 Oct 2013 12:16:16 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, Oct 10, 2013 at 12:04 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > I'm wondering if we can have a for_each_cpu() that only disables
> > preemption in the loop.
> 
> I think we'd generally want to have it be something the loop asks for.

Yeah, I was thinking of adding a new macro, not update for_each_cpu()
itself, so that only the locations that would require it would use it.

> 
> If the loop is just some kind of "gather statistics" thing, I don't
> think it's required. The cost per loop is so low (usually adding up a
> couple of words) that the downside drowns the upside.
> 
> And we could easily look at MAXSMP (or NR_CPUS) at compile-time, and
> not do it for common small values (although it looks like Fedora
> defaults to 128 CPU's for their distro kernels, which seems a bit
> excessive - too many by far for normal people, too few for the crazy
> big ones).

We could always make it a boot time option, and use alternates to
change the code as we do with spin_locks() and other code.

-- Steve (the lover of self modifying code)