Re: [PATCH 3/6] x86: Allow the user not to build hw_breakpoints

From: Frederic Weisbecker
Date: Thu Jul 21 2011 - 09:03:46 EST

On Thu, Jul 21, 2011 at 09:26:56AM +0200, Ingo Molnar wrote:
> * H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> > On 07/14/2011 08:03 AM, Frederic Weisbecker wrote:
> > > So that hw_breakpoints and perf can be not built on
> > > specific embedded systems.
> >
> > I want to emphasize I am very, very unhappy about this. It should
> > be possible to not build perf while still have breakpoints
> > available... breakpoints are way more important than perf.
> What we could indeed do is to separate out a 'core perf' portion that
> is necessary for hw-breakpoints to work fine, thus allowing for
> example the PMU drivers to be disabled.

That would still require a big chunk of perf.

> Otherwise we have expressed hw breakpoint APIs via perf events and
> that model is working well. Making hw-breakpoints a separate
> subsystem again with isolated (and partly duplicated) infrastructure
> would be a step back really.

I actually don't think it's working well. What we have with the current
design is the dependency to perf as a big midlayer that is apparently
convenient but actually induce some nasty things.

Just look how we need those ptrace_get_breakpoints() protection to deal
with perf exit path implementation for example. Or the need for archs
to translate arch ptrace breakpoint info into generic perf attrs.

I think we had to try the current design just to see if that could plug
nicely. But now that we have this for several releases, I can only conclude
that we should revert back to the design Prasad proposed, consisting in
having breakpoints a service used by perf but not the opposite.

For ptrace, all it takes is a generic hook in the preempt notifiers to
activate/deactivate breakpoints. I much prefer that to a big dependency
on a perf core midlayer.
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