Re: [PATCH v3] rcu: Remove impossible wakeup rcu GP kthread action from rcu_report_qs_rdp()

[Joel Fernandes]

On Fri, Jan 20, 2023 at 3:14 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Jan 18, 2023 at 10:07:14AM -0800, Paul E. McKenney wrote:
> > On Wed, Jan 18, 2023 at 03:30:14PM +0800, Zqiang wrote:
> > > When inovke rcu_report_qs_rdp(), if current CPU's rcu_data structure's ->
> > > grpmask has not been cleared from the corresponding rcu_node structure's
> > > ->qsmask, after that will clear and report quiescent state, but in this
> > > time, this also means that current grace period is not end, the current
> > > grace period is ongoing, because the rcu_gp_in_progress() currently return
> > > true, so for non-offloaded rdp, invoke rcu_accelerate_cbs() is impossible
> > > to return true.
> > >
> > > This commit therefore remove impossible rcu_gp_kthread_wake() calling.
> > >
> > > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> > > Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> >
> > Queued (wordsmithed as shown below, as always, please check) for further
> > testing and review, thank you both!
> >
> >                                                       Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit fbe3e300ec8b3edd2b8f84dab4dc98947cf71eb8
> > Author: Zqiang <qiang1.zhang@xxxxxxxxx>
> > Date:   Wed Jan 18 15:30:14 2023 +0800
> >
> >     rcu: Remove never-set needwake assignment from rcu_report_qs_rdp()
> >
> >     The rcu_accelerate_cbs() function is invoked by rcu_report_qs_rdp()
> >     only if there is a grace period in progress that is still blocked
> >     by at least one CPU on this rcu_node structure.  This means that
> >     rcu_accelerate_cbs() should never return the value true, and thus that
> >     this function should never set the needwake variable and in turn never
> >     invoke rcu_gp_kthread_wake().
> >
> >     This commit therefore removes the needwake variable and the invocation
> >     of rcu_gp_kthread_wake() in favor of a WARN_ON_ONCE() on the call to
> >     rcu_accelerate_cbs().  The purpose of this new WARN_ON_ONCE() is to
> >     detect situations where the system's opinion differs from ours.
> >
> >     Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> >     Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index b2c2045294780..7a3085ad0a7df 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1956,7 +1956,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> >  {
> >       unsigned long flags;
> >       unsigned long mask;
> > -     bool needwake = false;
> >       bool needacc = false;
> >       struct rcu_node *rnp;
> >
> > @@ -1988,7 +1987,12 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> >                * NOCB kthreads have their own way to deal with that...
> >                */
> >               if (!rcu_rdp_is_offloaded(rdp)) {
> > -                     needwake = rcu_accelerate_cbs(rnp, rdp);
> > +                     /*
> > +                      * The current GP has not yet ended, so it
> > +                      * should not be possible for rcu_accelerate_cbs()
> > +                      * to return true.  So complain, but don't awaken.
> > +                      */
> > +                     WARN_ON_ONCE(rcu_accelerate_cbs(rnp, rdp));
> >               } else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
> >                       /*
> >                        * ...but NOCB kthreads may miss or delay callbacks acceleration
> > @@ -2000,8 +2004,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
> >               rcu_disable_urgency_upon_qs(rdp);
> >               rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> >               /* ^^^ Released rnp->lock */
> > -             if (needwake)
> > -                     rcu_gp_kthread_wake();
>
> AFAICS, there is almost no compiler benefit of doing this, and zero runtime
> benefit of doing this. The WARN_ON_ONCE() also involves a runtime condition
> check of the return value of rcu_accelerate_cbs(), so you still have a
> branch. Yes, maybe slightly smaller code without the wake call, but I'm not
> sure that is worth it.
>
> And, if the opinion of system differs, its a bug anyway, so more added risk.
>
>
> >
> >               if (needacc) {
> >                       rcu_nocb_lock_irqsave(rdp, flags);
>
> And when needacc = true, rcu_accelerate_cbs_unlocked() tries to do a wake up
> anyway, so it is consistent with nocb vs !nocb.

Sorry, I mean "inconsistent".

Thanks,

 - Joel