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

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Date:	Tue, 31 Jan 2012 17:12:07 +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 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.

I didn't think I'd ever quote Bush, but my reaction to that is: 
'Bring it on!' ;-)

Really, we don't want *more* random delays in kernel code.

Thanks,

	Ingo
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