[PATCH v2] trace: tracing_event_filter: fast path when no subsystem filters

[Nicholas Lowell]

From: Nicholas Lowell <nlow...@lexmark.com>

If there are no filters in the event subsystem, then there's no
reason to continue and hit the potentially time consuming
tracepoint_synchronize_unregister function.  This should give
a speed up for initial disabling/configuring

Signed-off-by: Nicholas Lowell <nlow...@lexmark.com>
---

Notes:
    v2: switch from needless counting to boolean
        WARN_ON_ONCE if no preds but filter exists

 kernel/trace/trace_events_filter.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events_filter.c 
b/kernel/trace/trace_events_filter.c
index 33264e510d16..baa108f88032 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -1317,22 +1317,29 @@ void free_event_filter(struct event_filter *filter)
        __free_filter(filter);
 }
 
-static inline void __remove_filter(struct trace_event_file *file)
+static inline bool __remove_filter(struct trace_event_file *file)
 {
        filter_disable(file);
+       if (!file->filter)
+               return false;
+
        remove_filter_string(file->filter);
+       return true;
 }
 
-static void filter_free_subsystem_preds(struct trace_subsystem_dir *dir,
+static bool filter_free_subsystem_preds(struct trace_subsystem_dir *dir,
                                        struct trace_array *tr)
 {
        struct trace_event_file *file;
+       bool do_sync = false;
 
        list_for_each_entry(file, &tr->events, list) {
                if (file->system != dir)
                        continue;
-               __remove_filter(file);
+               if (__remove_filter(file))
+                       do_sync = true;
        }
+       return do_sync;
 }
 
 static inline void __free_subsystem_filter(struct trace_event_file *file)
@@ -2411,7 +2418,12 @@ int apply_subsystem_event_filter(struct 
trace_subsystem_dir *dir,
        }
 
        if (!strcmp(strstrip(filter_string), "0")) {
-               filter_free_subsystem_preds(dir, tr);
+               /* If nothing was freed, we do not need to sync */
+               if (!filter_free_subsystem_preds(dir, tr)) {
+                       if(!(WARN_ON_ONCE(system->filter)))
+                               goto out_unlock;
+               }
+
                remove_filter_string(system->filter);
                filter = system->filter;
                system->filter = NULL;
-- 
2.25.1