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

[Joel Fernandes]

On Fri, Sep 16, 2022 at 10:10 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> 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);

Never mind, this is still broken. I need to move the call site into
rcu_nocb_try_bypass() to be before where it releases the lock. I just
landed so I'll take a small break before getting back at it.

thanks,

 - Joel