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

[Zhang, Qiang1]

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);