Re: [PATCH V8 7/8] perf, x86: introduce PERF_RECORD_LOST_SAMPLES

[Peter Zijlstra]

So I changed it slightly to the below; changes are:

 - record 'lost' events to all set bits; after all we really do not
   know which event this sample belonged to, only logging to the first
   set bit seems 'wrong'.

 - dropped the @id field from the record, it is already included in the
   @sample_id values.


---
Subject: perf, x86: introduce PERF_RECORD_LOST_SAMPLES
From: Kan Liang <kan.liang@xxxxxxxxx>
Date: Wed, 6 May 2015 15:33:53 -0400

After enlarging the PEBS interrupt threshold, there may be some mixed up
PEBS samples which are discarded by kernel. This patch drives the kernel
to emit a PERF_RECORD_LOST_SAMPLES record with the number of possible
discards when it is impossible to demux the samples. It makes sure the
user is not left in the dark about such discards.

Cc: acme@xxxxxxxxxxxxx
Cc: eranian@xxxxxxxxxx
Cc: andi@xxxxxxxxxxxxxx
Cc: mingo@xxxxxxxxxx
Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1430940834-8964-8-git-send-email-kan.liang@xxxxxxxxx
---
 arch/x86/kernel/cpu/perf_event_intel_ds.c |   22 ++++++++++++++++----
 include/linux/perf_event.h                |    3 ++
 include/uapi/linux/perf_event.h           |   12 ++++++++++
 kernel/events/core.c                      |   33 ++++++++++++++++++++++++++++++
 4 files changed, 66 insertions(+), 4 deletions(-)

--- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
@@ -1124,8 +1124,9 @@ static void intel_pmu_drain_pebs_nhm(str
 	struct debug_store *ds = cpuc->ds;
 	struct perf_event *event;
 	void *base, *at, *top;
-	int bit;
 	short counts[MAX_PEBS_EVENTS] = {};
+	short error[MAX_PEBS_EVENTS] = {};
+	int bit, i;
 
 	if (!x86_pmu.pebs_active)
 		return;
@@ -1169,20 +1170,33 @@ static void intel_pmu_drain_pebs_nhm(str
 			/* slow path */
 			pebs_status = p->status & cpuc->pebs_enabled;
 			pebs_status &= (1ULL << MAX_PEBS_EVENTS) - 1;
-			if (pebs_status != (1 << bit))
+			if (pebs_status != (1 << bit)) {
+				for (i = 0; 1 < MAX_PEBS_EVENTS; i++) {
+					if (pebs_status & (1 << i))
+						error[i]++;
+				}
 				continue;
+			}
 		}
 		counts[bit]++;
 	}
 
 	for (bit = 0; bit < x86_pmu.max_pebs_events; bit++) {
-		if (counts[bit] == 0)
+		if ((counts[bit] == 0) && (error[bit] == 0))
 			continue;
+
 		event = cpuc->events[bit];
 		WARN_ON_ONCE(!event);
 		WARN_ON_ONCE(!event->attr.precise_ip);
 
-		__intel_pmu_pebs_event(event, iregs, base, top, bit, counts[bit]);
+		/* log dropped samples number */
+		if (error[bit])
+			perf_log_lost_samples(event, error[bit]);
+
+		if (counts[bit]) {
+			__intel_pmu_pebs_event(event, iregs, base,
+					       top, bit, counts[bit]);
+		}
 	}
 }
 
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -747,6 +747,9 @@ perf_event__output_id_sample(struct perf
 			     struct perf_output_handle *handle,
 			     struct perf_sample_data *sample);
 
+extern void
+perf_log_lost_samples(struct perf_event *event, u64 lost);
+
 static inline bool is_sampling_event(struct perf_event *event)
 {
 	return event->attr.sample_period != 0;
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -800,6 +800,18 @@ enum perf_event_type {
 	 */
 	PERF_RECORD_ITRACE_START		= 12,
 
+	/*
+	 * Records the dropped/lost sample number.
+	 *
+	 * struct {
+	 *	struct perf_event_header	header;
+	 *
+	 *	u64				lost;
+	 *	struct sample_id		sample_id;
+	 * };
+	 */
+	PERF_RECORD_LOST_SAMPLES		= 13,
+
 	PERF_RECORD_MAX,			/* non-ABI */
 };
 
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5947,6 +5947,39 @@ void perf_event_aux_event(struct perf_ev
 }
 
 /*
+ * Lost/dropped samples logging
+ */
+void perf_log_lost_samples(struct perf_event *event, u64 lost)
+{
+	struct perf_output_handle handle;
+	struct perf_sample_data sample;
+	int ret;
+
+	struct {
+		struct perf_event_header	header;
+		u64				lost;
+	} lost_samples_event = {
+		.header = {
+			.type = PERF_RECORD_LOST_SAMPLES,
+			.misc = 0,
+			.size = sizeof(lost_samples_event),
+		},
+		.lost		= lost,
+	};
+
+	perf_event_header__init_id(&lost_samples_event.header, &sample, event);
+
+	ret = perf_output_begin(&handle, event,
+				lost_samples_event.header.size);
+	if (ret)
+		return;
+
+	perf_output_put(&handle, lost_samples_event);
+	perf_event__output_id_sample(event, &handle, &sample);
+	perf_output_end(&handle);
+}
+
+/*
  * IRQ throttle logging
  */
 
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