linux-kernel - Re: [tip: core/rcu] rcu/tree: Mark the idle relevant functions noinstr

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Date:   Tue, 29 Sep 2020 10:34:54 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86 <x86@...nel.org>
Subject: Re: [tip: core/rcu] rcu/tree: Mark the idle relevant functions
 noinstr

On Tue, 29 Sep 2020 13:25:41 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, May 19, 2020 at 07:52:33PM -0000, tip-bot2 for Thomas Gleixner wrote:
> > @@ -979,7 +988,7 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
> >   * if the current CPU is not in its idle loop or is in an interrupt or
> >   * NMI handler, return true.
> >   */
> > -bool notrace rcu_is_watching(void)
> > +bool rcu_is_watching(void)
> >  {
> >  	bool ret;
> >    
> 
> This ^..
> 
> it is required because __ftrace_ops_list_func() /
> ftrace_ops_assist_func() call it outside of ftrace recursion, but only
> when FL_RCU, and perf happens to be the only user of that.
> 
> another morning wasted... :/

Those are fun to debug. :-p (I'll use this in another email).

Anyway, you bring up a good point. I should have this:

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 84f32dbc7be8..2d76eaaad4a7 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6993,16 +6993,14 @@ static void ftrace_ops_assist_func(unsigned long ip, unsigned long parent_ip,
 {
 	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;
 
 	preempt_disable_notrace();
 
-	op->func(ip, parent_ip, op, regs);
+	if (!(op->flags & FTRACE_OPS_FL_RCU) || rcu_is_watching())
+		op->func(ip, parent_ip, op, regs);
 
 	preempt_enable_notrace();
 	trace_clear_recursion(bit);



-- Steve