linux-kernel - Re: [PATCH v2] trace: Add x86 irq vector entry/exit tracepoints

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Date:	Fri, 8 Jul 2011 01:00:59 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	David Sharp <dhsharp@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michael Rubin <mrubin@...gle.com>, x86@...nel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jiaying Zhang <jiayingz@...gle.com>
Subject: Re: [PATCH v2] trace: Add x86 irq vector entry/exit tracepoints

On Thu, Jul 07, 2011 at 03:50:04PM -0700, David Sharp wrote:
> On Thu, Jul 7, 2011 at 2:57 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * H. Peter Anvin <hpa@...or.com> wrote:
> >
> >> Yes, it was much more of a generic concern.  However, it is very
> >> important that people have a correct idea about what the stability
> >> of something like tracepoint is -- or we'll end up in a situation
> >> where we can never change the kernel because anything is suddenly
> >> "user space visible."
> >
> > We've transitioned even ABI-assuming tracepoints in the past, so it's
> > not a big issue in practice. The reason is that this is an atypical
> > type of ABI: information is read-only exported, for observation
> > purposes.
> >
> > If the kernel changes in a fundamental way that removes a tracepoint
> > altogether, then there's nothing left to observe - so apps don't
> > break per se.
> >
> > So i've yet to see a single example of the kernel 'never being able
> > to change' due to a tracepoint. The worst we've seen in practice is
> > the inability to change a specific tracepoint (not the surrounding
> > kernel code - while preserving the information that is exposed) - so
> > the worst effect was limited to tracing itself - never to the
> > subsystem that it traces.
> >
> > Note that even in that (single known) example we were able to resolve
> > the problem (which was limited to the tracing subsystem) by adding
> > new tracepoints and thus phasing out the old ones.
> >
> > Thanks,
> >
> >        Ingo
> >
> 
> Thanks all for your thorough review. :) It sounds like there is some
> agreement now. I think this Steve is waiting for an Acked-by from an
> x86 maintainer to apply this patch. Are there any further objections
> or comments on the patch?

Yeah I have some comments on it. I just postponed my reply because I did
not have clear suggestions to propose back until now.

I'm posting that now.
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