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

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Message-ID: <20120131072216.1ce78e50@infradead.org>
Date:	Tue, 31 Jan 2012 07:22:16 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
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

On Tue, 31 Jan 2012 15:31:31 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> > 
> > 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.

it's not really a performance problem as it is an obvious "we have a
ton of back-to-back writers on a read-write lock that we have quite a
few readers for". Unless the writers back off a little, the readers are
going to get starved.


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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