We cannot assume that because hwc->idx == assign[i], we
can avoid reprogramming the counter in hw_perf_enable().
The event may have been scheduled out and another event
may have been programmed into this counter. Thus, we need
a more robust way of verifying if the counter still
contains config/data related to an event.
This patch adds a generation number to each counter on each
cpu. Using this mechanism we can verify reliabilty whether the
content of a counter corresponds to an event.
Signed-off-by: Stephane Eranian <eran...@google.com>
--
arch/x86/kernel/cpu/perf_event.c | 38 ++++++++++++++++++++++++++++++--------
include/linux/perf_event.h | 2 ++
2 files changed, 32 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 1846ead..c87bf1e 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -90,6 +90,7 @@ struct cpu_hw_events {
int n_events;
int n_added;
int assign[X86_PMC_IDX_MAX]; /* event to counter
assignment */
+ u64 tags[X86_PMC_IDX_MAX];
struct perf_event *event_list[X86_PMC_IDX_MAX]; /* in enabled
order */
};
@@ -1128,6 +1129,8 @@ static int __hw_perf_event_init(struct perf_event *event)
hwc->config = ARCH_PERFMON_EVENTSEL_INT;
hwc->idx = -1;
+ hwc->last_cpu = -1;
+ hwc->last_tag = ~0ULL;
/*
* Count user and OS events unless requested not to.
@@ -1443,11 +1446,14 @@ static int collect_events(struct cpu_hw_events *cpuc,
struct perf_event *leader,
return n;
}
-
static inline void x86_assign_hw_event(struct perf_event *event,
- struct hw_perf_event *hwc, int idx)
+ struct cpu_hw_events *cpuc, int i)
{
- hwc->idx = idx;
+ struct hw_perf_event *hwc = &event->hw;
+
+ hwc->idx = cpuc->assign[i];
+ hwc->last_cpu = smp_processor_id();
+ hwc->last_tag = ++cpuc->tags[i];
if (hwc->idx == X86_PMC_IDX_FIXED_BTS) {
hwc->config_base = 0;
@@ -1466,6 +1472,15 @@ static inline void x86_assign_hw_event(struct perf_event
*event,
}
}
+static inline int match_prev_assignment(struct hw_perf_event *hwc,
+ struct cpu_hw_events *cpuc,
+ int i)
+{
+ return hwc->idx == cpuc->assign[i]
+ && hwc->last_cpu == smp_processor_id()
+ && hwc->last_tag == cpuc->tags[i];
+}
+
static void __x86_pmu_disable(struct perf_event *event, struct cpu_hw_events
*cpuc);
void hw_perf_enable(void)
@@ -1494,8 +1509,15 @@ void hw_perf_enable(void)
event = cpuc->event_list[i];
hwc = &event->hw;
- if (hwc->idx == -1 || hwc->idx == cpuc->assign[i])
- continue;
+ /*
+ * we can avoid reprogramming counter if:
+ * - assigned same counter as last time
+ * - running on same CPU as last time
+ * - no other event has used the counter since
+ */
+ if (hwc->idx == -1
+ || match_prev_assignment(hwc, cpuc, i))
+ continue;
__x86_pmu_disable(event, cpuc);
@@ -1508,12 +1530,12 @@ void hw_perf_enable(void)
hwc = &event->hw;
if (hwc->idx == -1) {
- x86_assign_hw_event(event, hwc,
cpuc->assign[i]);
- x86_perf_event_set_period(event, hwc, hwc->idx);
+ x86_assign_hw_event(event, cpuc, i);
+ x86_perf_event_set_period(event, hwc, hwc->idx);
}
/*
* need to mark as active because x86_pmu_disable()
- * clear active_mask and eventsp[] yet it preserves
+ * clear active_mask and events[] yet it preserves
* idx
*/
set_bit(hwc->idx, cpuc->active_mask);
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 556b0f4..071a7db 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -478,9 +478,11 @@ struct hw_perf_event {
union {
struct { /* hardware */
u64 config;
+ u64 last_tag;
unsigned long config_base;
unsigned long event_base;
int idx;
+ int last_cpu;
};
struct { /* software */
s64 remaining;
------------------------------------------------------------------------------
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