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
