Commit 0d855354 authored Oct 26, 2012 by Michael Neuling Committed by Ingo Molnar Oct 30, 2012

perf, powerpc: Fix hw breakpoints returning -ENOSPC



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.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Michael Ellerman <michael@ellerman.id.au>
Cc: Jovi Zhang <bookjovi@gmail.com>
Cc: K Prasad <prasad@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1351268936-2956-1-git-send-email-fweisbec@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 35fd3dc5

kernel/events/hw_breakpoint.c

+7 −5

Original line number	Diff line number	Diff line
		@@ -111,14 +111,16 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
		* Count the number of breakpoints of the same type and same task.
		* The given event must be not on the list.
		*/
		static int task_bp_pinned(struct perf_event *bp, enum bp_type_idx type)
		static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
		{
		struct task_struct *tsk = bp->hw.bp_target;
		struct perf_event *iter;
		int count = 0;

		list_for_each_entry(iter, &bp_task_head, hw.bp_list) {
		if (iter->hw.bp_target == tsk && find_slot_idx(iter) == type)
		if (iter->hw.bp_target == tsk &&
		find_slot_idx(iter) == type &&
		cpu == iter->cpu)
		count += hw_breakpoint_weight(iter);
		}

		@@ -141,7 +143,7 @@ fetch_bp_busy_slots(struct bp_busy_slots slots, struct perf_event bp,
		if (!tsk)
		slots->pinned += max_task_bp_pinned(cpu, type);
		else
		slots->pinned += task_bp_pinned(bp, type);
		slots->pinned += task_bp_pinned(cpu, bp, type);
		slots->flexible = per_cpu(nr_bp_flexible[type], cpu);

		return;
		@@ -154,7 +156,7 @@ fetch_bp_busy_slots(struct bp_busy_slots slots, struct perf_event bp,
		if (!tsk)
		nr += max_task_bp_pinned(cpu, type);
		else
		nr += task_bp_pinned(bp, type);
		nr += task_bp_pinned(cpu, bp, type);

		if (nr > slots->pinned)
		slots->pinned = nr;
		@@ -188,7 +190,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu, bool enable,
		int old_idx = 0;
		int idx = 0;

		old_count = task_bp_pinned(bp, type);
		old_count = task_bp_pinned(cpu, bp, type);
		old_idx = old_count - 1;
		idx = old_idx + weight;