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

Mike Travis
2008-08-27T14:36:30Z

David Miller wrote:
> From: Nick Piggin <nickpiggin-/E1597aS9LT0CCvOHzKKcA@public.gmane.org>
> 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 :)
>
> Look at the sparc64 SMP code for how I solved the problem there.

I will, thanks!

Mike