linux-kernel - Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c

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Date:	Wed, 27 Aug 2008 01:44:57 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	nickpiggin@...oo.com.au
Cc:	travis@....com, davej@...hat.com, torvalds@...ux-foundation.org,
	Alan.Brunelle@...com, mingo@...e.hu, tglx@...utronix.de,
	rjw@...k.pl, linux-kernel@...r.kernel.org,
	kernel-testers@...r.kernel.org, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, rusty@...tcorp.com.au,
	suresh.b.siddha@...el.com, tony.luck@...el.com, steiner@....com,
	cl@...ux-foundation.org
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c -
 bisected

From: Nick Piggin <nickpiggin@...oo.com.au>
Date: Wed, 27 Aug 2008 17:47:14 +1000

> Yeah, I see. That's stupid isn't it? (Well, I guess it was completely
> sane when cpumasks were word sized ;))
> 
> Hopefully that accounts for a significant chunk...

There is a lot of indirect costs that are hard to see as well.

Two things a lot of these cross-call dispatch paths do is:

1) Clear self-cpu

2) AND with cpus_online

#1 can normally be a simple bit clear, but some places can also
implement this with something like "cpus_andn(X, cpumask_of_cpu(cpu))"

It's simply easier to move those two things down to the bottom of
the APIC programming code, they just loop over the cpumask doing
an expensive APIC I/O operation anyways, might as well overlap it
with these "skip self-cpu" and "skip not-online cpus" checks.

And oh yeah we get the stack wastage fixed too, isn't what what we
were talking about? :-)

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