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

[Frederic Weisbecker <fweisbec@gmail.com>]

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@elte.hu> wrote:
> >
> > * H. Peter Anvin <hpa@zytor.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.