Re: [PATCH rcu/next 2/3] rcu: Move trace_rcu_callback() before bypassing

[Joel Fernandes]

On Fri, Sep 16, 2022 at 01:09:49PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 15, 2022 at 12:14:18AM +0000, Joel Fernandes (Google) wrote:
> > If any CB is queued into the bypass list, then trace_rcu_callback() does
> > not show it. This makes it not clear when a callback was actually
> > queued, as you only end up getting a trace_rcu_invoke_callback() trace.
> > Fix it by moving trace_rcu_callback() before
> > trace_rcu_nocb_try_bypass().
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/rcu/tree.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 5ec97e3f7468..9fe581be8696 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2809,10 +2809,7 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  	}
> >  
> >  	check_cb_ovld(rdp);
> > -	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
> > -		return; // Enqueued onto ->nocb_bypass, so just leave.
> > -	// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
> > -	rcu_segcblist_enqueue(&rdp->cblist, head);
> > +
> >  	if (__is_kvfree_rcu_offset((unsigned long)func))
> >  		trace_rcu_kvfree_callback(rcu_state.name, head,
> >  					 (unsigned long)func,
> > @@ -2821,6 +2818,11 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
> >  		trace_rcu_callback(rcu_state.name, head,
> >  				   rcu_segcblist_n_cbs(&rdp->cblist));
> >  
> > +	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
> > +		return; // Enqueued onto ->nocb_bypass, so just leave.
> > +	// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
> > +	rcu_segcblist_enqueue(&rdp->cblist, head);
> > +
> >  	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
> >  
> >  	/* Go handle any RCU core processing required. */
> 
> Two subtle changes induced here:
> 
> * rcu_segcblist_n_cbs() is now read lockless. It's just tracing so no huge deal
>   but still, if this races with callbacks invocation, we may on some rare occasion
>   read stale numbers on traces while enqueuing (think about rcu_top for example)
> 
> * trace_rcu_callback() will now show the number of callbacks _before_ enqueuing
>   instead of _after_. Not sure if it matters, but sometimes tools rely on trace
>   events.
> 
> To avoid all that, how about a new trace_rcu_nocb_bypass() instead?

Great points, thanks much and you rock. How about something like the
following? That way we don't need to add yet another trace point:

---8<-----------------------

From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
Subject: [PATCH v2] rcu: Call trace_rcu_callback() also for bypassing

If any CB is queued into the bypass list, then trace_rcu_callback() does
not show it. This makes it not clear when a callback was actually
queued, as you only end up getting a trace_rcu_invoke_callback() trace.
Fix it by calling the tracing function even for bypass queue.

[Frederic: Hold lock while tracing]

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5ec97e3f7468..85609ccbb8ed 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2728,6 +2728,22 @@ static void check_cb_ovld(struct rcu_data *rdp)
 	raw_spin_unlock_rcu_node(rnp);
 }
 
+/*
+ * Trace RCU callback helper, call after enqueuing callback.
+ * The ->cblist must be locked when called.
+ */
+static void trace_rcu_callback_locked(struct rcu_head *head,
+				      struct rcu_data *rdp)
+{
+	if (__is_kvfree_rcu_offset((unsigned long)head->func))
+		trace_rcu_kvfree_callback(rcu_state.name, head,
+					 (unsigned long)head->func,
+					 rcu_segcblist_n_cbs(&rdp->cblist));
+	else
+		trace_rcu_callback(rcu_state.name, head,
+				   rcu_segcblist_n_cbs(&rdp->cblist));
+}
+
 /**
  * call_rcu() - Queue an RCU callback for invocation after a grace period.
  * @head: structure to be used for queueing the RCU updates.
@@ -2809,17 +2825,14 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
 	}
 
 	check_cb_ovld(rdp);
-	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags))
+
+	if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags)) {
+		trace_rcu_callback_locked(head, rdp);
 		return; // Enqueued onto ->nocb_bypass, so just leave.
+	}
 	// If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock.
 	rcu_segcblist_enqueue(&rdp->cblist, head);
-	if (__is_kvfree_rcu_offset((unsigned long)func))
-		trace_rcu_kvfree_callback(rcu_state.name, head,
-					 (unsigned long)func,
-					 rcu_segcblist_n_cbs(&rdp->cblist));
-	else
-		trace_rcu_callback(rcu_state.name, head,
-				   rcu_segcblist_n_cbs(&rdp->cblist));
+	trace_rcu_callback_locked(head, rdp);
 
 	trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
 
-- 
2.37.3.968.ga6b4b080e4-goog