Re: [PATCH v4] rcu/tree: Reduce wake up for synchronize_rcu() common case

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Mar 19, 2024 at 02:54:57PM -0400, Joel Fernandes (Google) wrote:
> 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>
> ---
> v1->v2: Rebase on paul/dev
> v2->v3: Additional optimization for wait_tail->next == NULL case.
> v3->v4: Apply clean ups from Vlad. Tested rcutorture all scenarios.
> ---
>  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 9fbb5ab57c84..f3193670fe42 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 +1706,34 @@ 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 except putting the second last
> +	 * wait head if no inflight-workers. If there are in-flight workers,
> +	 * they will remove the last wait head.
> +	 *
> +	 * Note that the ACQUIRE orders atomic access with list manipulation.
> +	 */
> +	if (wait_tail->next && wait_tail->next->next == NULL &&
> +	    rcu_sr_is_wait_head(wait_tail->next) &&
> +	    !atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
> +		rcu_sr_put_wait_head(wait_tail->next);
> +		wait_tail->next = NULL;
> +	}
> +
> +	/* Concurrent sr_normal_gp_cleanup work might observe this update. */
>  	ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
> +	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);
> +	if (wait_tail->next) {
> +		atomic_inc(&rcu_state.srs_cleanups_pending);
> +		if (!queue_work(sync_wq, &rcu_state.srs_cleanup_work))
> +			atomic_dec(&rcu_state.srs_cleanups_pending);
> +	}
>  }
>  
>  /*
> 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
> 
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

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Uladzislau Rezki