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Date: Wed, 5 Jun 2024 18:35:08 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com,
	rostedt@goodmis.org,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Uladzislau Rezki <urezki@gmail.com>
Subject: Re: [PATCH rcu 3/9] rcu/tree: Reduce wake up for synchronize_rcu()
 common case
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 <20240604222355.2370768-3-paulmck@kernel.org>
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Le Tue, Jun 04, 2024 at 03:23:49PM -0700, Paul E. McKenney a ?crit :
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> 
> In the synchronize_rcu() common case, we will have less than
> SR_MAX_USERS_WAKE_FROM_GP number of users per GP. Waking up the kworker
> is pointless just to free the last injected wait head since at that point,
> all the users have already been awakened.
> 
> Introduce a new counter to track this and prevent the wakeup in the
> common case.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/rcu/tree.c | 35 ++++++++++++++++++++++++++++++-----
>  kernel/rcu/tree.h |  1 +
>  2 files changed, 31 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 6ba36d9c09bde..2fe08e6186b4d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -96,6 +96,7 @@ static struct rcu_state rcu_state = {
>  	.ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
>  	.srs_cleanup_work = __WORK_INITIALIZER(rcu_state.srs_cleanup_work,
>  		rcu_sr_normal_gp_cleanup_work),
> +	.srs_cleanups_pending = ATOMIC_INIT(0),
>  };
>  
>  /* Dump rcu_node combining tree at boot to verify correct setup. */
> @@ -1633,8 +1634,11 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>  	 * the done tail list manipulations are protected here.
>  	 */
>  	done = smp_load_acquire(&rcu_state.srs_done_tail);
> -	if (!done)
> +	if (!done) {
> +		/* See comments below. */
> +		atomic_dec_return_release(&rcu_state.srs_cleanups_pending);

This condition is not supposed to happen. If the work is scheduled,
there has to be a wait_queue in rcu_state.srs_done_tail. And decrementing
may make things worse.

So this should be:

if (WARN_ON_ONCE(!done))
   return;

Thanks.