Messages in this thread | | | Date | Sat, 7 Nov 2009 20:52:09 +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 Sat, Nov 07, 2009 at 09:03:00PM +1100, Paul Mackerras wrote: > Frederic Weisbecker writes: > > > On Thu, Nov 05, 2009 at 10:59:44AM +1100, Paul Mackerras wrote: > > > 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 :) > > Suppose you have 4 breakpoint registers per cpu and there are two > pinned per-cpu breakpoint events, three non-pinned per-cpu breakpoint > events, and one pinned per-task breakpoint event. I believe your > constraints will allow that situation. > > What will happen is that the two pinned per-cpu breakpoint events will > use two of the hardware registers, and the three non-pinned per-cpu > breakpoint events will get round-robined onto the other two hardware > registers. The per-task breakpoint will never get to use a hardware > register, because the code in perf_event.c schedules per-cpu events > before it schedules per-task events (see for example > perf_event_task_tick()).
Oh! :-(
> We will have to make the event scheduling in kernel/perf_event.c a bit > more sophisticated before we can guarantee that a pinned breakpoint > event will always get to use a hardware register. > > Paul.
Ok, so the only solution for now (a part from fixing that into perf) is to consider the non-pinned events as being pinned in the constraints.
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