linux-kernel - Re: smp: Start up non-boot CPUs asynchronously

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Date:	Tue, 31 Jan 2012 15:31:31 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Milton Miller <miltonm@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	arjanvandeven@...il.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: smp: Start up non-boot CPUs asynchronously


* Arjan van de Ven <arjan@...radead.org> wrote:

> On Tue, 31 Jan 2012 13:52:32 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > The real fix would be to make the init code depend less on each 
> > other, i.e. have less hotplug lock dependencies. Or, if it's 
> > such a hot lock for a good reason, why does spinning on it slow 
> > down the boot process? It really shouldnt.
> 
> by inspection, anything that calls 
> get_online_cpus()/put_online_cpus() will block while a CPU is 
> coming up. This is used in things like kmem_cache_create()... 
> which is used about everywhere. (there's various other 
> places... more or less it's a requirement for using the 
> for_each_online_cpu() api correctly)

Still magic delays are not acceptable - we want to face any 
remaining performance problems head on, we want to understand 
and fix them correctly.

Thanks,

	Ingo
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