linux-kernel - Re: [PATCH v2] rcu: Remove impossible wakeup rcu GP kthread action from rcu_report_qs

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Message-ID: <Y8aql+vHNcdth0T1@lothringen>
Date:   Tue, 17 Jan 2023 15:03:03 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     paulmck@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Remove impossible wakeup rcu GP kthread action
 from rcu_report_qs_rdp()

On Tue, Jan 17, 2023 at 03:44:43PM +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@...el.com>
> ---
>  kernel/rcu/tree.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b2c204529478..c78d48482583 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 +1987,7 @@ 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);
> +			WARN_ON_ONCE(rcu_accelerate_cbs(rnp, rdp));

Please also add a comment explaining why we don't expect to have
to wake up the GP kthread.

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks!


>  		} else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
>  			/*
>  			 * ...but NOCB kthreads may miss or delay callbacks acceleration
> @@ -2000,8 +1999,6 @@ 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);
> -- 
> 2.25.1
>