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

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Date:	Wed, 1 Jul 2009 00:40:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, tj@...nel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-arch@...r.kernel.org, andi@...stfloor.org, hpa@...or.com,
	tglx@...utronix.de
Subject: Re: [PATCHSET] percpu: generalize first chunk allocators and improve lpage NUMA support

> How would you allow that guest to stay on 2 virtual CPUs but still 
> be able to hot-plug many other CPUs if the guest context rises above 
> its original CPU utilization?

(unless you're planning to rewrite lots of possible cpu users all over
the tree) -- the only way is to keep the percpu area small and preallocate. 

As long as the per cpu data size stays reasonable (not more than a 100-200k)
that's very doable. It probably won't work with 4096 guest CPUs without
wasting too much memory, but then I don't think we have any Hypervisor
that scales to that many CPUs anyways, so it's not the biggest
concern. For the 128CPU case it works (although i might need
to enlarge vmalloc area a bit on 32bit) 

Unfortunately a few debugging subsystems seem to currently eat
much more, but those just need to be fixed to only allocate
state for actually running CPUs, not just possible ones.

I suspect we need a scripts/percpubloat.pl

-Andi

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