perf/core: Define PERF_PMU_TXN_READ interface

Define a new PERF_PMU_TXN_READ interface to read a group of counters
at once.

        pmu->start_txn()                // Initialize before first event

        for each event in group
                pmu->read(event);       // Queue each event to be read

        rc = pmu->commit_txn()          // Read/update all queued counters

Note that we use this interface with all PMUs.  PMUs that implement this
interface use the ->read() operation to _queue_ the counters to be read
and use ->commit_txn() to actually read all the queued counters at once.

PMUs that don't implement PERF_PMU_TXN_READ ignore ->start_txn() and
->commit_txn() and continue to read counters one at a time.

Thanks to input from Peter Zijlstra.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/1441336073-22750-9-git-send-email-sukadev@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/perf_event.h

@@ -202,6 +202,7 @@ struct perf_event;
 #define PERF_EVENT_TXN 0x1
 
 #define PERF_PMU_TXN_ADD  0x1		/* txn to add/schedule event on PMU */
+#define PERF_PMU_TXN_READ 0x2		/* txn to read event group from PMU */
 
 /**
  * pmu::capabilities flags

kernel/events/core.c

@@ -3199,6 +3199,7 @@ static void __perf_event_read(void *info)
 	struct perf_event *sub, *event = data->event;
 	struct perf_event_context *ctx = event->ctx;
 	struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
+	struct pmu *pmu = event->pmu;
 
 	/*
 	 * If this is a task context, we need to check whether it is
@@ -3217,18 +3218,31 @@ static void __perf_event_read(void *info)
 	}
 
 	update_event_times(event);
-	if (event->state == PERF_EVENT_STATE_ACTIVE)
-		event->pmu->read(event);
+	if (event->state != PERF_EVENT_STATE_ACTIVE)
+		goto unlock;
 
-	if (!data->group)
+	if (!data->group) {
+		pmu->read(event);
+		data->ret = 0;
 		goto unlock;
+	}
+
+	pmu->start_txn(pmu, PERF_PMU_TXN_READ);
+
+	pmu->read(event);
 
 	list_for_each_entry(sub, &event->sibling_list, group_entry) {
 		update_event_times(sub);
-		if (sub->state == PERF_EVENT_STATE_ACTIVE)
+		if (sub->state == PERF_EVENT_STATE_ACTIVE) {
+			/*
+			 * Use sibling's PMU rather than @event's since
+			 * sibling could be on different (eg: software) PMU.
+			 */
 			sub->pmu->read(sub);
+		}
 	}
-	data->ret = 0;
+
+	data->ret = pmu->commit_txn(pmu);
 
 unlock:
 	raw_spin_unlock(&ctx->lock);