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From: Cyrill Gorcunov on 5 Aug 2010 11:10
In case if last active performance counter is not overflowed at
moment of NMI being triggered by another counter, the irq statistics
may miss an update stage. As a more serious consequence -- apic quirk
may not be triggered so apic lvt entry stay masked.

Tested-by: Lin Ming <ming.m.lin(a)intel.com>
Signed-off-by: Cyrill Gorcunov <gorcunov(a)openvz.org>
CC: Lin Ming <ming.m.lin(a)intel.com>
CC: Stephane Eranian <eranian(a)google.com>
CC: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
CC: Ingo Molnar <mingo(a)elte.hu>
CC: Frederic Weisbecker <fweisbec(a)gmail.com>
---
arch/x86/kernel/cpu/perf_event_p4.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

Index: linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
=====================================================================
--- linux-2.6.git.orig/arch/x86/kernel/cpu/perf_event_p4.c
+++ linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
@@ -656,6 +656,7 @@ static int p4_pmu_handle_irq(struct pt_r
cpuc = &__get_cpu_var(cpu_hw_events);

for (idx = 0; idx < x86_pmu.num_counters; idx++) {
+ int overflow;

if (!test_bit(idx, cpuc->active_mask))
continue;
@@ -666,12 +667,14 @@ static int p4_pmu_handle_irq(struct pt_r
WARN_ON_ONCE(hwc->idx != idx);

/* it might be unflagged overflow */
- handled = p4_pmu_clear_cccr_ovf(hwc);
+ overflow = p4_pmu_clear_cccr_ovf(hwc);

val = x86_perf_event_update(event);
- if (!handled && (val & (1ULL << (x86_pmu.cntval_bits - 1))))
+ if (!overflow && (val & (1ULL << (x86_pmu.cntval_bits - 1))))
continue;

+ handled += overflow;
+
/* event overflow for sure */
data.period = event->hw.last_period;

@@ -687,7 +690,7 @@ static int p4_pmu_handle_irq(struct pt_r
inc_irq_stat(apic_perf_irqs);
}

- return handled;
+ return handled > 0;
}

/*
--
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