perf, x86, nmi: Move LVT un-masking into irq handlers

It was noticed that P4 machines were generating double NMIs for
each perf event.  These extra NMIs lead to 'Dazed and confused'
messages on the screen.

I tracked this down to a P4 quirk that said the overflow bit had
to be cleared before re-enabling the apic LVT mask.  My first
attempt was to move the un-masking inside the perf nmi handler
from before the chipset NMI handler to after.

This broke Nehalem boxes that seem to like the unmasking before
the counters themselves are re-enabled.

In order to keep this change simple for 2.6.39, I decided to
just simply move the apic LVT un-masking to the beginning of all
the chipset NMI handlers, with the exception of Pentium4's to
fix the double NMI issue.

Later on we can move the un-masking to later in the handlers to
save a number of 'extra' NMIs on those particular chipsets.

I tested this change on a P4 machine, an AMD machine, a Nehalem
box, and a core2quad box.  'perf top' worked correctly along
with various other small 'perf record' runs.  Anything high
stress breaks all the machines but that is a different problem.

Thanks to various people for testing different versions of this
patch.

Reported-and-tested-by: Shaun Ruffell <sruffell@digium.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Link: http://lkml.kernel.org/r/1303900353-10242-1-git-send-email-dzickus@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
CC: Cyrill Gorcunov <gorcunov@gmail.com>

3 files changed, 33 insertions, 6 deletions

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index fac0654021b8..e638689279d3 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1288,6 +1288,16 @@ static int x86_pmu_handle_irq(struct pt_regs *regs)
 
         cpuc = &__get_cpu_var(cpu_hw_events);
 
+        /*
+         * Some chipsets need to unmask the LVTPC in a particular spot
+         * inside the nmi handler.  As a result, the unmasking was pushed
+         * into all the nmi handlers.
+         *
+         * This generic handler doesn't seem to have any issues where the
+         * unmasking occurs so it was left at the top.
+         */
+        apic_write(APIC_LVTPC, APIC_DM_NMI);
+
         for (idx = 0; idx < x86_pmu.num_counters; idx++) {
                 if (!test_bit(idx, cpuc->active_mask)) {
                         /*
@@ -1374,8 +1384,6 @@ perf_event_nmi_handler(struct notifier_block *self,
                 return NOTIFY_DONE;
         }
 
-        apic_write(APIC_LVTPC, APIC_DM_NMI);
-
         handled = x86_pmu.handle_irq(args->regs);
         if (!handled)
                 return NOTIFY_DONE;
diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
index 9ae4a2aa7398..e61539b07d2c 100644
--- a/arch/x86/kernel/cpu/perf_event_intel.c
+++ b/arch/x86/kernel/cpu/perf_event_intel.c
@@ -933,6 +933,16 @@ static int intel_pmu_handle_irq(struct pt_regs *regs)
 
         cpuc = &__get_cpu_var(cpu_hw_events);
 
+        /*
+         * Some chipsets need to unmask the LVTPC in a particular spot
+         * inside the nmi handler.  As a result, the unmasking was pushed
+         * into all the nmi handlers.
+         *
+         * This handler doesn't seem to have any issues with the unmasking
+         * so it was left at the top.
+         */
+        apic_write(APIC_LVTPC, APIC_DM_NMI);
+
         intel_pmu_disable_all();
         handled = intel_pmu_drain_bts_buffer();
         status = intel_pmu_get_status();
diff --git a/arch/x86/kernel/cpu/perf_event_p4.c b/arch/x86/kernel/cpu/perf_event_p4.c
index d1f77e2934a1..e93fcd55fae1 100644
--- a/arch/x86/kernel/cpu/perf_event_p4.c
+++ b/arch/x86/kernel/cpu/perf_event_p4.c
@@ -950,11 +950,20 @@ static int p4_pmu_handle_irq(struct pt_regs *regs)
                         x86_pmu_stop(event, 0);
         }
 
-        if (handled) {
-                /* p4 quirk: unmask it again */
-                apic_write(APIC_LVTPC, apic_read(APIC_LVTPC) & ~APIC_LVT_MASKED);
+        if (handled)
                 inc_irq_stat(apic_perf_irqs);
-        }
+
+        /*
+         * When dealing with the unmasking of the LVTPC on P4 perf hw, it has
+         * been observed that the OVF bit flag has to be cleared first _before_
+         * the LVTPC can be unmasked.
+         *
+         * The reason is the NMI line will continue to be asserted while the OVF
+         * bit is set.  This causes a second NMI to generate if the LVTPC is
+         * unmasked before the OVF bit is cleared, leading to unknown NMI
+         * messages.
+         */
+        apic_write(APIC_LVTPC, APIC_DM_NMI);
 
         return handled;
 }