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

[Nick Piggin]

On Wednesday 27 August 2008 17:05, David Miller wrote:
> From: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
> 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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