perf_counter: Log irq_period changes

For the dynamic irq_period code, log whenever we change the period so that
analyzing code can normalize the event flow.

[ Impact: add new feature to allow more precise profiling ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <20090520102553.298769743@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -257,6 +257,14 @@ enum perf_event_type {
 	 */
 	PERF_EVENT_COMM			= 3,
 
+	/*
+	 * struct {
+	 * 	struct perf_event_header	header;
+	 * 	u64				irq_period;
+	 * };
+	 */
+	PERF_EVENT_PERIOD		= 4,
+
 	/*
 	 * When header.misc & PERF_EVENT_MISC_OVERFLOW the event_type field
 	 * will be PERF_RECORD_*

kernel/perf_counter.c

@@ -1046,7 +1046,9 @@ int perf_counter_task_enable(void)
 	return 0;
 }
 
-void perf_adjust_freq(struct perf_counter_context *ctx)
+static void perf_log_period(struct perf_counter *counter, u64 period);
+
+static void perf_adjust_freq(struct perf_counter_context *ctx)
 {
 	struct perf_counter *counter;
 	u64 irq_period;
@@ -1072,6 +1074,8 @@ void perf_adjust_freq(struct perf_counter_context *ctx)
 		if (!irq_period)
 			irq_period = 1;
 
+		perf_log_period(counter, irq_period);
+
 		counter->hw.irq_period = irq_period;
 		counter->hw.interrupts = 0;
 	}
@@ -2406,6 +2410,40 @@ void perf_counter_munmap(unsigned long addr, unsigned long len,
 	perf_counter_mmap_event(&mmap_event);
 }
 
+/*
+ *
+ */
+
+static void perf_log_period(struct perf_counter *counter, u64 period)
+{
+	struct perf_output_handle handle;
+	int ret;
+
+	struct {
+		struct perf_event_header	header;
+		u64				time;
+		u64				period;
+	} freq_event = {
+		.header = {
+			.type = PERF_EVENT_PERIOD,
+			.misc = 0,
+			.size = sizeof(freq_event),
+		},
+		.time = sched_clock(),
+		.period = period,
+	};
+
+	if (counter->hw.irq_period == period)
+		return;
+
+	ret = perf_output_begin(&handle, counter, sizeof(freq_event), 0, 0);
+	if (ret)
+		return;
+
+	perf_output_put(&handle, freq_event);
+	perf_output_end(&handle);
+}
+
 /*
  * Generic counter overflow handling.
  */