Re: [PATCH] perf, x86: try to handle unknown nmis with running perfctrs

[Don Zickus <dzickus@redhat.com>]

On Mon, Aug 09, 2010 at 09:48:29PM +0200, Robert Richter wrote:
> On 06.08.10 10:21:31, Don Zickus wrote:
> > On Fri, Aug 06, 2010 at 08:52:03AM +0200, Robert Richter wrote:
> 
> > > I was playing around with it yesterday trying to fix this. My idea is
> > > to skip an unkown nmi if the privious nmi was a *handled* perfctr
> > 
> > You might want to add a little more logic that says *handled* _and_ had
> > more than one perfctr trigger.  Most of the time only one perfctr is
> > probably triggering, so you might be eating unknown_nmi's needlessly.
> > 
> > Just a thought.
> 
> Yes, that's true. It could be implemented on top of the patch below.

I did, but the changes basically revert the bulk of your patch.

> 
> > 
> > > nmi. I will probably post an rfc patch early next week.
> 
> Here it comes:
> 
> From d2739578199d881ae6a9537c1b96a0efd1cdea43 Mon Sep 17 00:00:00 2001
> From: Robert Richter <robert.richter@amd.com>
> Date: Thu, 5 Aug 2010 16:19:59 +0200
> Subject: [PATCH] perf, x86: try to handle unknown nmis with running perfctrs

On top of Robert's patch:
(compiled tested only because I don't have a fancy button to trigger
unknown nmis)

>From 548cf5148f47618854a0eff22b1d55db71b6f8fc Mon Sep 17 00:00:00 2001
From: Don Zickus <dzickus@redhat.com>
Date: Tue, 10 Aug 2010 16:40:03 -0400
Subject: [PATCH] perf, x86: only skip NMIs when multiple perfctrs trigger

A small optimization on top of Robert's patch that limits the
skipping of NMI's to cases where we detect multiple perfctr events
have happened.

Signed-off-by: Don Zickus <dzickus@redhat.com>

---
 arch/x86/kernel/cpu/perf_event.c |   34 ++++++++++++++++++++--------------
 1 files changed, 20 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index c3cd159..066046d 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1154,7 +1154,7 @@ static int x86_pmu_handle_irq(struct pt_regs *regs)
 		/*
 		 * event overflow
 		 */
-		handled		= 1;
+		handled		+= 1;
 		data.period	= event->hw.last_period;
 
 		if (!x86_perf_event_set_period(event))
@@ -1200,7 +1200,7 @@ void perf_events_lapic_init(void)
 	apic_write(APIC_LVTPC, APIC_DM_NMI);
 }
 
-static DEFINE_PER_CPU(unsigned int, perfctr_handled);
+static DEFINE_PER_CPU(unsigned int, perfctr_skip);
 
 static int __kprobes
 perf_event_nmi_handler(struct notifier_block *self,
@@ -1208,8 +1208,7 @@ perf_event_nmi_handler(struct notifier_block *self,
 {
 	struct die_args *args = __args;
 	struct pt_regs *regs;
-	unsigned int this_nmi;
-	unsigned int prev_nmi;
+	int handled = 0;
 
 	if (!atomic_read(&active_events))
 		return NOTIFY_DONE;
@@ -1229,14 +1228,11 @@ perf_event_nmi_handler(struct notifier_block *self,
 		 * was handling a perfctr. Otherwise we pass it and
 		 * let the kernel handle the unknown nmi.
 		 *
-		 * Note: this could be improved if we drop unknown
-		 * NMIs only if we handled more than one perfctr in
-		 * the previous NMI.
 		 */
-		this_nmi = percpu_read(irq_stat.__nmi_count);
-		prev_nmi = __get_cpu_var(perfctr_handled);
-		if (this_nmi == prev_nmi + 1)
+		if (__get_cpu_var(perfctr_skip)){
+			__get_cpu_var(perfctr_skip) -=1;
 			return NOTIFY_STOP;
+		}
 		return NOTIFY_DONE;
 	default:
 		return NOTIFY_DONE;
@@ -1246,11 +1242,21 @@ perf_event_nmi_handler(struct notifier_block *self,
 
 	apic_write(APIC_LVTPC, APIC_DM_NMI);
 
-	if (!x86_pmu.handle_irq(regs))
+	handled = x86_pmu.handle_irq(regs);
+	if (!handled)
+		/* not our NMI */
 		return NOTIFY_DONE;
-
-	/* handled */
-	__get_cpu_var(perfctr_handled) = percpu_read(irq_stat.__nmi_count);
+	else if (handled > 1)
+		/*
+		 * More than one perfctr triggered.  This could have
+		 * caused a second NMI that we must now skip because
+		 * we have already handled it.  Remember it.
+		 *
+		 * NOTE: We have no way of knowing if a second NMI was
+		 * actually triggered, so we may accidentally skip a valid
+		 * unknown nmi later.
+		 */
+		__get_cpu_var(perfctr_skip) +=1;
 
 	return NOTIFY_STOP;
 }
-- 
1.7.2