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

Don Zickus
2010-08-10T20:49:21Z

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