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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 13 Jul 2016 11:42:35 -0700
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Subject: Re: [PATCH v5 14/32] x86/mm/64: Enable vmapped stacks
To: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
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On Wed, Jul 13, 2016 at 12:53 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Andy Lutomirski <luto@kernel.org> wrote:
>
>> This allows x86_64 kernels to enable vmapped stacks.  There are a
>> couple of interesting bits.
>
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -92,6 +92,7 @@ config X86
>>       select HAVE_ARCH_TRACEHOOK
>>       select HAVE_ARCH_TRANSPARENT_HUGEPAGE
>>       select HAVE_EBPF_JIT                    if X86_64
>> +     select HAVE_ARCH_VMAP_STACK             if X86_64
>
> So what is the performance impact?

Seems to be a very slight speedup (0.5 µs or so) on my silly benchmark
(pthread_create, pthread_join in a loop).  It should be a small
slowdown on workloads that create many threads all at once, thus
defeating the stack cache.  It should be a *large* speedup on any
workload that would trigger compaction on clone() to satisfy the
high-order allocation.

>
> Because I think we should consider enabling this feature by default on x86 - but
> the way it's selected here it will be default-off.
>
> On the plus side: the debuggability and reliability improvements are real and
> making it harder for exploits to use kernel stack overflows is a nice bonus as
> well. There's two performance effects:

Agreed.  At the very least, I want to wait until after net-next gets
pulled to flip the default to y.  I'm also a bit concerned about more
random driver issues that I haven't found yet.  I suppose we could
flip the default to y for a few -rc releases and see what, if
anything, shakes loose.

--Andy