Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 20 Jan 2023 03:14:55 +0000 | From | Joel Fernandes <> | Subject | Re: [PATCH v3] rcu: Remove impossible wakeup rcu GP kthread action from rcu_report_qs_rdp() |
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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@intel.com> > > Reviewed-by: Frederic Weisbecker <frederic@kernel.org> > > 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@intel.com> > 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@intel.com> > Reviewed-by: Frederic Weisbecker <frederic@kernel.org> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > > 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.
So I am not a fan of this change. ;-)
thanks,
- Joel
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