Messages in this thread |  | | | Date | Thu, 5 Nov 2009 12:09:52 +0100 | | From | Frederic Weisbecker <> | | Subject | Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events |
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On Thu, Nov 05, 2009 at 10:59:44AM +1100, Paul Mackerras wrote: > Frederic Weisbecker writes: > > > This patch rebase the implementation of the breakpoints API on top of > > perf events instances. > > > > Each breakpoints are now perf events that handle the > > register scheduling, thread/cpu attachment, etc.. > > What I haven't managed to understand yet is how you provide reliable > breakpoints for debugging purposes. If I'm debugging a program and I > have set a breakpoint, I'll be very unhappy if the breakpoint should > trigger but doesn't because the perf_event infrastructure has decided > it can't schedule that breakpoint in. If the breakpoint isn't going > to work then I want to know that at the time that I set it.
That won't happen because of the set of constraints we have. We never overcommit the debug register resources, except in the case of non-pinned counter, but that's in their nature :)
> We can go some distance towards that with the pinned attribute, but > not far enough. The pinned attribute doesn't guarantee that the event > will always be scheduled in, it just says that we'll do our best to > schedule it in, and if we can't, we'll put the event into error state > so that the user knows we didn't manage to schedule it in. But > there's no way to get back to gdb and tell it that a breakpoint that > it had previously successfully created is no longer working. > > Also, we don't currently fail the creation of a pinned event if it > would conflict with another pinned event already created in the same > context. We would need to do something like that if we want to use > pinned events for debugging breakpoints (as distinct from breakpoints > for performance monitoring purposes, for which it matters less if they > are sometimes not scheduled in). > > And then there's the question of whether a per-cpu pinned breakpoint > event conflicts with a per-task pinned breakpoint event if you only > have one breakpoint register (as is the case on Power server CPUs). > Plus the fact that we don't currently give per-task pinned events > priority over per-cpu non-pinned events (perhaps that would be a good > idea anyway). > > Paul.
All that is already handled by the constraints.
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