linux-kernel - Re: [PATCHSET] percpu: generalize first chunk allocators and improve lpage NUMA support

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Message-ID: <20090625091939.GS6760@one.firstfloor.org>
Date:	Thu, 25 Jun 2009 11:19:39 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-arch@...r.kernel.org, mingo@...e.hu,
	tglx@...utronix.de, cl@...ux-foundation.org
Subject: Re: [PATCHSET] percpu: generalize first chunk allocators and improve lpage NUMA support

On Wed, Jun 24, 2009 at 05:13:37PM -0700, H. Peter Anvin wrote:
> Andi Kleen wrote:
> > 
> > Haven't read the new patches, but per cpu data always was sized
> > for all possible CPUs.
> > 
> >> and N is large, what did it cost?
> > 
> >> And what are reasonable values of N?
> > 
> > N should normally not be large anymore, since num_possible_cpus() 
> > is sized based on firmware information now.
> > 
> 
> *Ahem* virtual machines *ahem*...

And? Even there's not that big typically. 

The traditional problem was just for 128 NR_CPUs kernel were nothing
was sized based on machine capacity.

Also on large systems the VMs shouldn't be sized for full capacity.

> 
> I have discussed this with Tejun, and the plan is to allocate the percpu
> information when a processor is first brought online (but not removed
> when it is offlined again.)  It's a real problem for 32-bit VMs, so it's
> more important than you'd think.

You have to rewrite all code that does for_each_possible_cpu (x) 
in initialization then to use callbacks. It would be a gigantic change
all over the tree.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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