perf/x86: Only print PMU state when also WARN()'ing

intel_pmu_handle_irq() has a warning in it if it does too many
loops.  It is a WARN_ONCE(), but the perf_event_print_debug()
call beneath it is unconditional. For the first warning, you get
a nice backtrace and message, but subsequent ones just dump the
PMU state with no leading messages.  I doubt this is what was
intended.

This patch will only print the PMU state when paired with the
WARN_ON() text.  It effectively open-codes WARN_ONCE()'s
one-time-only logic.

My suspicion is that the code really just wants to make sure we
do not sit in the loop and spit out a warning for every loop
iteration after the 100th.  From what I've seen, this is very
unlikely to happen since we also clear the PMU state.

After this patch, instead of seeing the PMU state dumped each
time, you will just see:

	[57494.894540] perf_event_intel: clearing PMU state on CPU#129
	[57579.539668] perf_event_intel: clearing PMU state on CPU#10
	[57587.137762] perf_event_intel: clearing PMU state on CPU#134
	[57623.039912] perf_event_intel: clearing PMU state on CPU#114
	[57644.559943] perf_event_intel: clearing PMU state on CPU#118
	...
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20130530174559.0DB049F4@viggo.jf.intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/perf_event_intel.c

@@ -1188,8 +1188,12 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
 again:
 	intel_pmu_ack_status(status);
 	if (++loops > 100) {
-		WARN_ONCE(1, "perfevents: irq loop stuck!\n");
-		perf_event_print_debug();
+		static bool warned = false;
+		if (!warned) {
+			WARN(1, "perfevents: irq loop stuck!\n");
+			perf_event_print_debug();
+			warned = true;
+		}
 		intel_pmu_reset();
 		goto done;
 	}