linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Message-ID: <20131011060944.GB4975@gmail.com>
Date:	Fri, 11 Oct 2013 08:09:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Oct 10, 2013 at 12:04 PM, Steven Rostedt <rostedt@...dmis.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.
> 
> 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).

Ubuntu has it at 256, so I guess Fedora is even a bit conservative ;-)

Thanks,

	Ingo
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