tracing: Add 'snapshot' event trigger command

Add 'snapshot' event_command.  snapshot event triggers are added by
the user via this command in a similar way and using practically the
same syntax as the analogous 'snapshot' ftrace function command, but
instead of writing to the set_ftrace_filter file, the snapshot event
trigger is written to the per-event 'trigger' files:

    echo 'snapshot' > .../somesys/someevent/trigger

The above command will turn on snapshots for someevent i.e. whenever
someevent is hit, a snapshot will be done.

This also adds a 'count' version that limits the number of times the
command will be invoked:

    echo 'snapshot:N' > .../somesys/someevent/trigger

Where N is the number of times the command will be invoked.

The above command will snapshot N times for someevent i.e. whenever
someevent is hit N times, a snapshot will be done.

Also adds a new tracing_alloc_snapshot() function - the existing
tracing_snapshot_alloc() function is a special version of
tracing_snapshot() that also does the snapshot allocation - the
snapshot triggers would like to be able to do just the allocation but
not take a snapshot; the existing tracing_snapshot_alloc() in turn now
also calls tracing_alloc_snapshot() underneath to do that allocation.

Link: http://lkml.kernel.org/r/c9524dd07ce01f9dcbd59011290e0a8d5b47d7ad.1382622043.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
[ fix up from kbuild test robot <fengguang.wu@intel.com report ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 30 insertions, 3 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9d20cd9..59bf5b5 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -595,6 +595,28 @@ void free_snapshot(struct trace_array *tr)
 }
 
 /**
+ * tracing_alloc_snapshot - allocate snapshot buffer.
+ *
+ * This only allocates the snapshot buffer if it isn't already
+ * allocated - it doesn't also take a snapshot.
+ *
+ * This is meant to be used in cases where the snapshot buffer needs
+ * to be set up for events that can't sleep but need to be able to
+ * trigger a snapshot.
+ */
+int tracing_alloc_snapshot(void)
+{
+	struct trace_array *tr = &global_trace;
+	int ret;
+
+	ret = alloc_snapshot(tr);
+	WARN_ON(ret < 0);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(tracing_alloc_snapshot);
+
+/**
  * trace_snapshot_alloc - allocate and take a snapshot of the current buffer.
  *
  * This is similar to trace_snapshot(), but it will allocate the
@@ -607,11 +629,10 @@ void free_snapshot(struct trace_array *tr)
  */
 void tracing_snapshot_alloc(void)
 {
-	struct trace_array *tr = &global_trace;
 	int ret;
 
-	ret = alloc_snapshot(tr);
-	if (WARN_ON(ret < 0))
+	ret = tracing_alloc_snapshot();
+	if (ret < 0)
 		return;
 
 	tracing_snapshot();
@@ -623,6 +644,12 @@ void tracing_snapshot(void)
 	WARN_ONCE(1, "Snapshot feature not enabled, but internal snapshot used");
 }
 EXPORT_SYMBOL_GPL(tracing_snapshot);
+int tracing_alloc_snapshot(void)
+{
+	WARN_ONCE(1, "Snapshot feature not enabled, but snapshot allocation used");
+	return -ENODEV;
+}
+EXPORT_SYMBOL_GPL(tracing_alloc_snapshot);
 void tracing_snapshot_alloc(void)
 {
 	/* Give warning */