Re: [PATCH] x86/xen: avoid updating TLS descriptors if they haven't changed

Konrad Rzeszutek Wilk
2012-06-14T16:56:59Z

On Thu, Jun 14, 2012 at 03:52:19PM +0100, David Vrabel wrote:
> On 07/06/12 18:01, David Vrabel wrote:
> > From: David Vrabel <david.vrabel@citrix.com>
> >
> > When switching tasks in a Xen PV guest, avoid updating the TLS
> > descriptors if they haven't changed. This improves the speed of
> > context switches by almost 10% as much of the time the descriptors are
> > the same or only one is different.
> >
> > The descriptors written into the GDT by Xen are modified from the
> > values passed in the update_descriptor hypercall so we keep shadow
> > copies of the three TLS descriptors to compare against.
> >
> > lmbench3 test Before After Improvement
> > --------------------------------------------
> > lat_ctx -s 32 24 7.19 6.52 9%
> > lat_pipe 12.56 11.66 7%
> >
> > Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> > ---
> > I note that the comment in asm/desc_defs.h says the 'a' and 'b' fields
> > in desc_struct as deprecated but there seems to be no suitable
> > alternatives.
>
> ping? Any opinion on this patch from the x86 side? If it's okay can we
> get an ack so Konrad can take the patch via his tree.

It breaks my all my bootup tests - so NACK until at least that is fixed.
I think I sent you the whole serial log - is there something else that would help
narrow it down?