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

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Date:	Thu, 14 Jun 2012 12:49:26 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/xen: avoid updating TLS descriptors if they haven't
 changed

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@...rix.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@...rix.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?
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