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    SubjectRe: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events
    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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