linux-kernel - RE: [PATCH] x86, perf, bts: disable BTS from Nehalem to Ivy Bridge

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Date:	Thu, 24 Jan 2013 16:17:26 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Kettenis <kettenis@....org>,
	Pedro Alves <palves@...hat.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	"gdb-patches@...rceware.org" <gdb-patches@...rceware.org>
Subject: RE: [PATCH] x86, perf, bts: disable BTS from Nehalem to Ivy Bridge

> -----Original Message-----
> From: gdb-patches-owner@...rceware.org [mailto:gdb-patches-owner@...rceware.org] On Behalf Of Ingo Molnar
> Sent: Thursday, January 24, 2013 4:47 PM

> > Starting with Nehalem, the BTS "from" information may in some cases be
> > incorrect (AAJ122).
> >
> > This has been detected while adding branch tracing support to gdb, where it
> > results in sporadic test fails.
> >
> > Disable BTS support on Nehalem, Westmere, Sandy Bridge, and Ivy Bridge.
> 
> But the failures are rare, so the BTS data is still correct
> statistically, by and large, right? So why not just keep it
> as-is and teach tooling to be more robust about implausible
> trace entries? It has to be ready for that eventuality *anyway*.

Yes, the fails are rare. They are frequent enough, however, to cause sporadic test fails.

The problem is that you can't reliably detect such fails. In the worst case, GDB would show wrong but plausible trace. Typically, GDB just shows garbage (i.e. impossible trace) in such cases.

I'm afraid one might endlessly debate whether the feature as-is is useful or not. It is working fine on Atom so I chose to disable it on core.

The GDB folks would rather not disable the feature inside GDB; they prefer that it is disabled in the kernel. There will be an additional check inside GDB to support older kernel versions.

Regards,
markus.
Intel GmbH
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