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    SubjectRe: powerpc/perf: hw breakpoints return ENOSPC
    Date
    Michael Neuling <mikey@neuling.org> wrote:

    > Frederic Weisbecker <fweisbec@gmail.com> wrote:
    >
    > > On Thu, Aug 16, 2012 at 02:23:54PM +1000, Michael Neuling wrote:
    > > > Hi,
    > > >
    > > > I've been trying to get hardware breakpoints with perf to work on POWER7
    > > > but I'm getting the following:
    > > >
    > > > % perf record -e mem:0x10000000 true
    > > >
    > > > Error: sys_perf_event_open() syscall returned with 28 (No space left on device). /bin/dmesg may provide additional information.
    > > >
    > > > Fatal: No CONFIG_PERF_EVENTS=y kernel support configured?
    > > >
    > > > true: Terminated
    > > >
    > > > (FWIW adding -a and it works fine)
    > > >
    > > > Debugging it seems that __reserve_bp_slot() is returning ENOSPC because
    > > > it thinks there are no free breakpoint slots on this CPU.
    > > >
    > > > I have a 2 CPUs, so perf userspace is doing two perf_event_open syscalls
    > > > to add a counter to each CPU [1]. The first syscall succeeds but the
    > > > second is failing.
    > > >
    > > > On this second syscall, fetch_bp_busy_slots() sets slots.pinned to be 1,
    > > > despite there being no breakpoint on this CPU. This is because the call
    > > > the task_bp_pinned, checks all CPUs, rather than just the current CPU.
    > > > POWER7 only has one hardware breakpoint per CPU (ie. HBP_NUM=1), so we
    > > > return ENOSPC.
    > > >
    > > > The following patch fixes this by checking the associated CPU for each
    > > > breakpoint in task_bp_pinned. I'm not familiar with this code, so it's
    > > > provided as a reference to the above issue.
    > > >
    > > > Mikey
    > > >
    > > > 1. not sure why it doesn't just do one syscall and specify all CPUs, but
    > > > that's another issue. Using two syscalls should work.
    > >
    > > This patch seems to make sense. I'll try it and run some tests.
    > > Can I have your Signed-off-by ?

    Frederic,

    Did you ever get to testing or integrating this patch?

    Mikey

    > Of course...
    >
    > Signed-off-by: Michael Neuling <mikey@neuling.org>


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