[PATCH 8/9] perf/ftrace: Check for rcu_is_watching() in callback function

Steven Rostedt
2020-10-28T22:17:32Z

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a ftrace callback requires "rcu_is_watching", then it adds the
FTRACE_OPS_FL_RCU flag and it will not be called if RCU is not "watching".
But this means that it will use a trampoline when called, and this slows
down the function tracing a tad. By checking rcu_is_watching() from within
the callback, it no longer needs the RCU flag set in the ftrace_ops and it
can be safely called directly.

Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/trace_event_perf.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index fd58d83861d8..a2b9fddb8148 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -441,6 +441,9 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
int rctx;
int bit;

+ if (!rcu_is_watching())
+ return;
+
if ((unsigned long)ops->private != smp_processor_id())
return;

@@ -484,7 +487,6 @@ static int perf_ftrace_function_register(struct perf_event *event)
{
struct ftrace_ops *ops = &event->ftrace_ops;

- ops->flags = FTRACE_OPS_FL_RCU;
ops->func = perf_ftrace_function_call;
ops->private = (void *)(unsigned long)nr_cpu_ids;

--
2.28.0