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

[Joel Fernandes <joel@joelfernandes.org>]

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