Re: [PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

[Peter Zijlstra]

On Tue, Jan 24, 2023 at 05:12:14PM +0000, Mark Rutland wrote:
> On Tue, Jan 24, 2023 at 03:44:35PM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 23, 2023 at 05:07:53PM -0500, Steven Rostedt wrote:
> > 
> > > Actually, perhaps we can just add this, and all you need to do is create
> > > and set CONFIG_NO_RCU_TRACING (or some other name).
> > 
> > Elsewhere I've used CONFIG_ARCH_WANTS_NO_INSTR for this.
> 
> Yes please; if we use CONFIG_ARCH_WANTS_NO_INSTR then arm64 will get this "for
> free" once we add the missing checks (which I assume we need) in our ftrace_prepare_return().
> 
> > Anyway, I took it for a spin and it .... doesn't seems to do the job.
> > 
> > With my patch the first splat is
> > 
> >   "RCU not on for: cpuidle_poll_time+0x0/0x70"
> > 
> > While with yours I seems to get the endless:
> > 
> >   "WARNING: suspicious RCU usage"
> > 
> > thing. Let me see if I can figure out where it goes side-ways.
> 
> Hmmm... for WARN_ONCE() don't we need to wake RCU first also? I thought we
> needed that at least for the printk machinery?

OK, the below seems to work nice for me -- although I'm still on a
hacked up printk, but the recursive RCU not watching fail seems to be
tamed.

Ofc. Paul might have an opinion on this glorious bodge ;-)

---

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index c303f7a114e9..d48cd92d2364 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -135,6 +135,21 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
 # define do_ftrace_record_recursion(ip, pip)	do { } while (0)
 #endif
 
+#ifdef CONFIG_ARCH_WANTS_NO_INSTR
+# define trace_warn_on_no_rcu(ip)					\
+	({								\
+		bool __ret = !rcu_is_watching();			\
+		if (__ret && !trace_recursion_test(TRACE_RECORD_RECURSION_BIT)) { \
+			trace_recursion_set(TRACE_RECORD_RECURSION_BIT); \
+			WARN_ONCE(true, "RCU not on for: %pS\n", (void *)ip); \
+			trace_recursion_clear(TRACE_RECORD_RECURSION_BIT); \
+		}							\
+		__ret;							\
+	})
+#else
+# define trace_warn_on_no_rcu(ip)	false
+#endif
+
 /*
  * Preemption is promised to be disabled when return bit >= 0.
  */
@@ -144,6 +159,9 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
 	unsigned int val = READ_ONCE(current->trace_recursion);
 	int bit;
 
+	if (trace_warn_on_no_rcu(ip))
+		return -1;
+
 	bit = trace_get_context_bit() + start;
 	if (unlikely(val & (1 << bit))) {
 		/*
diff --git a/lib/bug.c b/lib/bug.c
index c223a2575b72..0a10643ea168 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -47,6 +47,7 @@
 #include <linux/sched.h>
 #include <linux/rculist.h>
 #include <linux/ftrace.h>
+#include <linux/context_tracking.h>
 
 extern struct bug_entry __start___bug_table[], __stop___bug_table[];
 
@@ -153,7 +154,7 @@ struct bug_entry *find_bug(unsigned long bugaddr)
 	return module_find_bug(bugaddr);
 }
 
-enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
+static enum bug_trap_type __report_bug(unsigned long bugaddr, struct pt_regs *regs)
 {
 	struct bug_entry *bug;
 	const char *file;
@@ -209,6 +210,30 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
 	return BUG_TRAP_TYPE_BUG;
 }
 
+enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
+{
+	enum bug_trap_type ret;
+	bool rcu = false;
+
+#ifdef CONFIG_CONTEXT_TRACKING_IDLE
+	/*
+	 * Horrible hack to shut up recursive RCU isn't watching fail since
+	 * lots of the actual reporting also relies on RCU.
+	 */
+	if (!rcu_is_watching()) {
+		rcu = true;
+		ct_state_inc(RCU_DYNTICKS_IDX);
+	}
+#endif
+
+	ret = __report_bug(bugaddr, regs);
+
+	if (rcu)
+		ct_state_inc(RCU_DYNTICKS_IDX);
+
+	return ret;
+}
+
 static void clear_once_table(struct bug_entry *start, struct bug_entry *end)
 {
 	struct bug_entry *bug;