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 17:47:14 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Miller <davem@...emloft.net>
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

On Wednesday 27 August 2008 17:05, David Miller wrote:
> From: Nick Piggin <nickpiggin@...oo.com.au>
> Date: Wed, 27 Aug 2008 16:54:32 +1000
>
> > 5% is a pretty nasty performance hit... what sort of benchmarks are we
> > talking about here?
> >
> > I just made some pretty crazy changes to the VM to get "only" around 5
> > or so % performance improvement in some workloads.
> >
> > What places are making heavy use of cpumasks that causes such a slowdown?
> > Hopefully callers can mostly be improved so they don't need to use
> > cpumasks for common cases.
>
> It's almost certainly from the cross-call dispatch call chain.
>
> As just one example, just to do a TLB flush mm->cpu_vm_mask probably
> gets passed around as an aggregate two or three times on the way down
> to the APIC programming code on x86.  That's two or three 512 byte
> copies on the stack :)

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...
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