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. > Thanks Paul, this is more clear for me😊. Thanks Zqiang > 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(); > > if (needacc) { > rcu_nocb_lock_irqsave(rdp, flags);