Re: [PATCH] ftrace: Remove use of control list and ops

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 1 Dec 2015 16:57:44 +0100
Jiri Olsa <jolsa@redhat.com> wrote:


> > Hmm, I thought that I forced the list function when RCU or PER_CPU
> > was set. Oh wait. I have CONFIG_PREEMPT set, which will change the
> > logic slightly. I'm guessing you have PREEMPT_VOLUNTARY set. I'll try
> > that out.  
> 
> yep, but the trampoline has separate code path
> to set the ops func

Right, but that only gets called for DYNAMIC ops (which perf is) when
CONFIG_PREEMPT is not set. What about this patch instead? (on top of my
other one)

It's a lot like yours, but instead of creating a new helper function, I
just reuse the recurs_func instead.

-- Steve

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 8b22e681baf8..05ed87d06bb0 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5230,20 +5230,29 @@ static void ftrace_ops_no_ops(unsigned long ip, unsigned long parent_ip)
 
 /*
  * If there's only one function registered but it does not support
- * recursion, this function will be called by the mcount trampoline.
- * This function will handle recursion protection.
+ * recursion, needs RCU protection and/or requires per cpu handling, then
+ * this function will be called by the mcount trampoline.
  */
-static void ftrace_ops_recurs_func(unsigned long ip, unsigned long parent_ip,
+static void ftrace_ops_assist_func(unsigned long ip, unsigned long parent_ip,
 				   struct ftrace_ops *op, struct pt_regs *regs)
 {
 	int bit;
 
+	if ((op->flags & FTRACE_OPS_FL_RCU) && !rcu_is_watching())
+		return;
+
 	bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
 	if (bit < 0)
 		return;
 
-	op->func(ip, parent_ip, op, regs);
+	preempt_disable_notrace();
 
+	if (!(op->flags & FTRACE_OPS_FL_PER_CPU) ||
+	    ftrace_function_local_disabled(op)) {
+		op->func(ip, parent_ip, op, regs);
+	}
+
+	preempt_enable_notrace();
 	trace_clear_recursion(bit);
 }
 
@@ -5261,12 +5270,12 @@ static void ftrace_ops_recurs_func(unsigned long ip, unsigned long parent_ip,
 ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops)
 {
 	/*
-	 * If the func handles its own recursion, call it directly.
-	 * Otherwise call the recursion protected function that
-	 * will call the ftrace ops function.
+	 * If the function does not handle recursion, needs to be RCU safe,
+	 * or does per cpu logic, then we need to call the assist handler.
 	 */
-	if (!(ops->flags & FTRACE_OPS_FL_RECURSION_SAFE))
-		return ftrace_ops_recurs_func;
+	if (!(ops->flags & FTRACE_OPS_FL_RECURSION_SAFE) ||
+	    ops->flags & (FTRACE_OPS_FL_RCU | FTRACE_OPS_FL_PER_CPU))
+		return ftrace_ops_assist_func;
 
 	return ops->func;
 }