perf_counter: x86: Fix throttling

If counters are disabled globally when a perfcounter IRQ/NMI hits,
and if we throttle in that case, we'll promote the '0' value to
the next lapic IRQ and disable all perfcounters at that point,
permanently ...

Fix it.

[ Impact: fix hung perfcounters under load ]
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_counter.c

@@ -765,8 +765,13 @@ static int intel_pmu_handle_irq(struct pt_regs *regs, int nmi)
 	/*
 	 * Restore - do not reenable when global enable is off or throttled:
 	 */
-	if (++cpuc->interrupts < PERFMON_MAX_INTERRUPTS)
-		intel_pmu_restore_all(cpuc->throttle_ctrl);
+	if (cpuc->throttle_ctrl) {
+		if (++cpuc->interrupts < PERFMON_MAX_INTERRUPTS) {
+			intel_pmu_restore_all(cpuc->throttle_ctrl);
+		} else {
+			pr_info("CPU#%d: perfcounters: max interrupt rate exceeded! Throttle on.\n", smp_processor_id());
+		}
+	}
 
 	return ret;
 }
@@ -817,11 +822,16 @@ void perf_counter_unthrottle(void)
 
 	cpuc = &__get_cpu_var(cpu_hw_counters);
 	if (cpuc->interrupts >= PERFMON_MAX_INTERRUPTS) {
-		if (printk_ratelimit())
-			printk(KERN_WARNING "perfcounters: max interrupts exceeded!\n");
+		pr_info("CPU#%d: perfcounters: throttle off.\n", smp_processor_id());
+
+		/*
+		 * Clear them before re-enabling irqs/NMIs again:
+		 */
+		cpuc->interrupts = 0;
 		hw_perf_restore(cpuc->throttle_ctrl);
+	} else {
+		cpuc->interrupts = 0;
 	}
-	cpuc->interrupts = 0;
 }
 
 void smp_perf_counter_interrupt(struct pt_regs *regs)