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From: Uladzislau Rezki <urezki@gmail.com>
Date: Mon, 18 Mar 2024 19:58:10 +0100
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org, frederic@kernel.org, boqun.feng@gmail.com,
	urezki@gmail.com, neeraj.iitr10@gmail.com, rcu@vger.kernel.org,
	rostedt@goodmis.org, "Paul E. McKenney" <paulmck@kernel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang1211@gmail.com>
Subject: Re: [PATCH v2 rcu/dev 1/2] rcu/tree: Reduce wake up for
 synchronize_rcu() common case
Message-ID: <ZfiOwtPfnxXW4JX3@pc636>
References: <20240308224439.281349-1-joel@joelfernandes.org>
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In-Reply-To: <20240308224439.281349-1-joel@joelfernandes.org>

Hello, Joel!

Sorry for late checking, see below few comments:

> 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>
> ---
> Rebased on paul/dev of today.
> 
>  kernel/rcu/tree.c | 36 +++++++++++++++++++++++++++++++-----
>  kernel/rcu/tree.h |  1 +
>  2 files changed, 32 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9fbb5ab57c84..bd29fe3c76bf 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. */
> @@ -1642,8 +1643,11 @@ 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);
>  		return;
> +	}
>  
>  	WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
>  	head = done->next;
> @@ -1666,6 +1670,9 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>  
>  		rcu_sr_put_wait_head(rcu);
>  	}
> +
> +	/* Order list manipulations with atomic access. */
> +	atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
>  }
>  
>  /*
> @@ -1673,7 +1680,7 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>   */
>  static void rcu_sr_normal_gp_cleanup(void)
>  {
> -	struct llist_node *wait_tail, *next, *rcu;
> +	struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
>  	int done = 0;
>  
>  	wait_tail = rcu_state.srs_wait_tail;
> @@ -1699,16 +1706,35 @@ static void rcu_sr_normal_gp_cleanup(void)
>  			break;
>  	}
>  
> -	// concurrent sr_normal_gp_cleanup work might observe this update.
> -	smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> +	/*
> +	 * Fast path, no more users to process. Remove the last wait head
> +	 * if no inflight-workers. If there are in-flight workers, let them
> +	 * remove the last wait head.
> +	 */
> +	WARN_ON_ONCE(!rcu);
>
This assumption is not correct. An "rcu" can be NULL in fact. 

>  	ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
>  
> +	if (rcu && rcu_sr_is_wait_head(rcu) && rcu->next == NULL &&
> +		/* Order atomic access with list manipulation. */
> +		!atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
> +		wait_tail->next = NULL;
> +		rcu_sr_put_wait_head(rcu);
> +		smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> +		return;
> +	}
> +
> +	/* Concurrent sr_normal_gp_cleanup work might observe this update. */
> +	smp_store_release(&rcu_state.srs_done_tail, wait_tail);
> +
>  	/*
>  	 * We schedule a work in order to perform a final processing
>  	 * of outstanding users(if still left) and releasing wait-heads
>  	 * added by rcu_sr_normal_gp_init() call.
>  	 */
> -	queue_work(sync_wq, &rcu_state.srs_cleanup_work);
> +	atomic_inc(&rcu_state.srs_cleanups_pending);
> +	if (!queue_work(sync_wq, &rcu_state.srs_cleanup_work)) {
> +		atomic_dec(&rcu_state.srs_cleanups_pending);
> +	}
>  }
No need an extra "{}" pair.

>  
>  /*
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index bae7925c497f..affcb92a358c 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -420,6 +420,7 @@ struct rcu_state {
>  	struct llist_node *srs_done_tail; /* ready for GP users. */
>  	struct sr_wait_node srs_wait_nodes[SR_NORMAL_GP_WAIT_HEAD_MAX];
>  	struct work_struct srs_cleanup_work;
> +	atomic_t srs_cleanups_pending; /* srs inflight worker cleanups. */
>  };
>  
>  /* Values for rcu_state structure's gp_flags field. */
> -- 
> 2.34.1
>