Message-ID: <579f86e0-e03e-4ab3-9a85-a62064bcf2a1@paulmck-laptop>
Date: Tue, 19 Dec 2023 17:37:56 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: RCU <rcu@...r.kernel.org>, Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v3 4/7] rcu: Improve handling of synchronize_rcu() users

On Tue, Nov 28, 2023 at 09:00:30AM +0100, Uladzislau Rezki (Sony) wrote:
> From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
> 
> Currently, processing of the next batch of rcu_synchronize nodes
> for the new grace period, requires doing a llist reversal operation
> to find the tail element of the list. This can be a very costly
> operation (high number of cache misses) for a long list.
> 
> To address this, this patch introduces a "dummy-wait-node" entity.
> At every grace period init, a new wait node is added to the llist.
> This wait node is used as wait tail for this new grace period.
> 
> This allows lockless additions of new rcu_synchronize nodes in the
> rcu_sr_normal_add_req(), while the cleanup work executes and does
> the progress. The dummy nodes are removed on next round of cleanup
> work execution.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>

This says that Uladzislau created the patch and that Neeraj
acted as maintainer.  I am guessing that you both worked on it,
in which case is should have the Co-developed-by tags as shown in
Documentation/process/submitting-patches.rst.  Could you please update
these to reflect the actual origin?

One question below toward the end.  There are probably others that I
should be asking, but I have to start somewhere.  ;-)

						Thanx, Paul

> ---
>  kernel/rcu/tree.c | 270 +++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 233 insertions(+), 37 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 975621ef40e3..d7b48996825f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1384,25 +1384,173 @@ static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  }
>  
> +#define SR_NORMAL_GP_WAIT_HEAD_MAX 5
> +
> +struct sr_wait_node {
> +	atomic_t inuse;
> +	struct llist_node node;
> +};
> +
>  /*
> - * There are three lists for handling synchronize_rcu() users.
> - * A first list corresponds to new coming users, second for users
> - * which wait for a grace period and third is for which a grace
> - * period is passed.
> + * There is a single llist, which is used for handling
> + * synchronize_rcu() users' enqueued rcu_synchronize nodes.
> + * Within this llist, there are two tail pointers:
> + *
> + * wait tail: Tracks the set of nodes, which need to
> + *            wait for the current GP to complete.
> + * done tail: Tracks the set of nodes, for which grace
> + *            period has elapsed. These nodes processing
> + *            will be done as part of the cleanup work
> + *            execution by a kworker.
> + *
> + * At every grace period init, a new wait node is added
> + * to the llist. This wait node is used as wait tail
> + * for this new grace period. Given that there are a fixed
> + * number of wait nodes, if all wait nodes are in use
> + * (which can happen when kworker callback processing
> + * is delayed) and additional grace period is requested.
> + * This means, a system is slow in processing callbacks.
> + *
> + * TODO: If a slow processing is detected, a first node
> + * in the llist should be used as a wait-tail for this
> + * grace period, therefore users which should wait due
> + * to a slow process are handled by _this_ grace period
> + * and not next.
> + *
> + * Below is an illustration of how the done and wait
> + * tail pointers move from one set of rcu_synchronize nodes
> + * to the other, as grace periods start and finish and
> + * nodes are processed by kworker.
> + *
> + *
> + * a. Initial llist callbacks list:
> + *
> + * +----------+           +--------+          +-------+
> + * |          |           |        |          |       |
> + * |   head   |---------> |   cb2  |--------->| cb1   |
> + * |          |           |        |          |       |
> + * +----------+           +--------+          +-------+
> + *
> + *
> + *
> + * b. New GP1 Start:
> + *
> + *                    WAIT TAIL
> + *                      |
> + *                      |
> + *                      v
> + * +----------+     +--------+      +--------+        +-------+
> + * |          |     |        |      |        |        |       |
> + * |   head   ------> wait   |------>   cb2  |------> |  cb1  |
> + * |          |     | head1  |      |        |        |       |
> + * +----------+     +--------+      +--------+        +-------+
> + *
> + *
> + *
> + * c. GP completion:
> + *
> + * WAIT_TAIL == DONE_TAIL
> + *
> + *                   DONE TAIL
> + *                     |
> + *                     |
> + *                     v
> + * +----------+     +--------+      +--------+        +-------+
> + * |          |     |        |      |        |        |       |
> + * |   head   ------> wait   |------>   cb2  |------> |  cb1  |
> + * |          |     | head1  |      |        |        |       |
> + * +----------+     +--------+      +--------+        +-------+
> + *
> + *
> + *
> + * d. New callbacks and GP2 start:
> + *
> + *                    WAIT TAIL                          DONE TAIL
> + *                      |                                 |
> + *                      |                                 |
> + *                      v                                 v
> + * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
> + * |          |     |      |    |      |    |      |    |     |    |     |    |     |
> + * |   head   ------> wait |--->|  cb4 |--->| cb3  |--->|wait |--->| cb2 |--->| cb1 |
> + * |          |     | head2|    |      |    |      |    |head1|    |     |    |     |
> + * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
> + *
> + *
> + *
> + * e. GP2 completion:
> + *
> + * WAIT_TAIL == DONE_TAIL
> + *                   DONE TAIL
> + *                      |
> + *                      |
> + *                      v
> + * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
> + * |          |     |      |    |      |    |      |    |     |    |     |    |     |
> + * |   head   ------> wait |--->|  cb4 |--->| cb3  |--->|wait |--->| cb2 |--->| cb1 |
> + * |          |     | head2|    |      |    |      |    |head1|    |     |    |     |
> + * +----------+     +------+    +------+    +------+    +-----+    +-----+    +-----+
> + *
> + *
> + * While the llist state transitions from d to e, a kworker
> + * can start executing rcu_sr_normal_gp_cleanup_work() and
> + * can observe either the old done tail (@c) or the new
> + * done tail (@e). So, done tail updates and reads need
> + * to use the rel-acq semantics. If the concurrent kworker
> + * observes the old done tail, the newly queued work
> + * execution will process the updated done tail. If the
> + * concurrent kworker observes the new done tail, then
> + * the newly queued work will skip processing the done
> + * tail, as workqueue semantics guarantees that the new
> + * work is executed only after the previous one completes.
> + *
> + * f. kworker callbacks processing complete:
> + *
> + *
> + *                   DONE TAIL
> + *                     |
> + *                     |
> + *                     v
> + * +----------+     +--------+
> + * |          |     |        |
> + * |   head   ------> wait   |
> + * |          |     | head2  |
> + * +----------+     +--------+
> + *
>   */
>  static struct sr_normal_state {
>  	struct llist_head srs_next;	/* request a GP users. */
> -	struct llist_head srs_wait;	/* wait for GP users. */
> -	struct llist_head srs_done;	/* ready for GP users. */
> -
> -	/*
> -	 * In order to add a batch of nodes to already
> -	 * existing srs-done-list, a tail of srs-wait-list
> -	 * is maintained.
> -	 */
> -	struct llist_node *srs_wait_tail;
> +	struct llist_node *srs_wait_tail; /* wait for GP users. */
> +	struct llist_node *srs_done_tail; /* ready for GP users. */
> +	struct sr_wait_node srs_wait_nodes[SR_NORMAL_GP_WAIT_HEAD_MAX];
>  } sr;
>  
> +static bool rcu_sr_is_wait_head(struct llist_node *node)
> +{
> +	return &(sr.srs_wait_nodes)[0].node <= node &&
> +		node <= &(sr.srs_wait_nodes)[SR_NORMAL_GP_WAIT_HEAD_MAX - 1].node;
> +}
> +
> +static struct llist_node *rcu_sr_get_wait_head(void)
> +{
> +	struct sr_wait_node *sr_wn;
> +	int i;
> +
> +	for (i = 0; i < SR_NORMAL_GP_WAIT_HEAD_MAX; i++) {
> +		sr_wn = &(sr.srs_wait_nodes)[i];
> +
> +		if (!atomic_cmpxchg_acquire(&sr_wn->inuse, 0, 1))
> +			return &sr_wn->node;
> +	}
> +
> +	return NULL;
> +}
> +
> +static void rcu_sr_put_wait_head(struct llist_node *node)
> +{
> +	struct sr_wait_node *sr_wn = container_of(node, struct sr_wait_node, node);
> +	atomic_set_release(&sr_wn->inuse, 0);
> +}
> +
>  /* Disabled by default. */
>  static int rcu_normal_wake_from_gp;
>  module_param(rcu_normal_wake_from_gp, int, 0644);
> @@ -1423,14 +1571,44 @@ static void rcu_sr_normal_complete(struct llist_node *node)
>  
>  static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>  {
> -	struct llist_node *done, *rcu, *next;
> +	struct llist_node *done, *rcu, *next, *head;
>  
> -	done = llist_del_all(&sr.srs_done);
> +	/*
> +	 * This work execution can potentially execute
> +	 * while a new done tail is being updated by
> +	 * grace period kthread in rcu_sr_normal_gp_cleanup().
> +	 * So, read and updates of done tail need to
> +	 * follow acq-rel semantics.
> +	 *
> +	 * Given that wq semantics guarantees that a single work
> +	 * cannot execute concurrently by multiple kworkers,
> +	 * the done tail list manipulations are protected here.
> +	 */
> +	done = smp_load_acquire(&sr.srs_done_tail);
>  	if (!done)
>  		return;
>  
> -	llist_for_each_safe(rcu, next, done)
> -		rcu_sr_normal_complete(rcu);
> +	WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
> +	head = done->next;
> +	done->next = NULL;
> +
> +	/*
> +	 * The dummy node, which is pointed to by the
> +	 * done tail which is acq-read above is not removed
> +	 * here.  This allows lockless additions of new
> +	 * rcu_synchronize nodes in rcu_sr_normal_add_req(),
> +	 * while the cleanup work executes. The dummy
> +	 * nodes is removed, in next round of cleanup
> +	 * work execution.
> +	 */
> +	llist_for_each_safe(rcu, next, head) {
> +		if (!rcu_sr_is_wait_head(rcu)) {
> +			rcu_sr_normal_complete(rcu);
> +			continue;
> +		}
> +
> +		rcu_sr_put_wait_head(rcu);
> +	}
>  }
>  static DECLARE_WORK(sr_normal_gp_cleanup, rcu_sr_normal_gp_cleanup_work);
>  
> @@ -1439,43 +1617,56 @@ static DECLARE_WORK(sr_normal_gp_cleanup, rcu_sr_normal_gp_cleanup_work);
>   */
>  static void rcu_sr_normal_gp_cleanup(void)
>  {
> -	struct llist_node *head, *tail;
> +	struct llist_node *wait_tail;
>  
> -	if (llist_empty(&sr.srs_wait))
> +	wait_tail = sr.srs_wait_tail;
> +	if (wait_tail == NULL)
>  		return;
>  
> -	tail = READ_ONCE(sr.srs_wait_tail);
> -	head = __llist_del_all(&sr.srs_wait);
> +	sr.srs_wait_tail = NULL;
> +	ASSERT_EXCLUSIVE_WRITER(sr.srs_wait_tail);
>  
> -	if (head) {
> -		/* Can be not empty. */
> -		llist_add_batch(head, tail, &sr.srs_done);
> +	// concurrent sr_normal_gp_cleanup work might observe this update.
> +	smp_store_release(&sr.srs_done_tail, wait_tail);
> +	ASSERT_EXCLUSIVE_WRITER(sr.srs_done_tail);
> +
> +	if (wait_tail)
>  		queue_work(system_highpri_wq, &sr_normal_gp_cleanup);
> -	}
>  }
>  
>  /*
>   * Helper function for rcu_gp_init().
>   */
> -static void rcu_sr_normal_gp_init(void)
> +static bool rcu_sr_normal_gp_init(void)
>  {
> -	struct llist_node *head, *tail;
> +	struct llist_node *first;
> +	struct llist_node *wait_head;
> +	bool start_new_poll = false;
>  
> -	if (llist_empty(&sr.srs_next))
> -		return;
> +	first = READ_ONCE(sr.srs_next.first);
> +	if (!first || rcu_sr_is_wait_head(first))
> +		return start_new_poll;
> +
> +	wait_head = rcu_sr_get_wait_head();
> +	if (!wait_head) {
> +		// Kick another GP to retry.
> +		start_new_poll = true;
> +		return start_new_poll;
> +	}
>  
> -	tail = llist_del_all(&sr.srs_next);
> -	head = llist_reverse_order(tail);
> +	/* Inject a wait-dummy-node. */
> +	llist_add(wait_head, &sr.srs_next);
>  
>  	/*
> -	 * A waiting list of GP should be empty on this step,
> -	 * since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
> +	 * A waiting list of rcu_synchronize nodes should be empty on
> +	 * this step, since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
>  	 * rolls it over. If not, it is a BUG, warn a user.
>  	 */
> -	WARN_ON_ONCE(!llist_empty(&sr.srs_wait));
> +	WARN_ON_ONCE(sr.srs_wait_tail != NULL);
> +	sr.srs_wait_tail = wait_head;
> +	ASSERT_EXCLUSIVE_WRITER(sr.srs_wait_tail);
>  
> -	WRITE_ONCE(sr.srs_wait_tail, tail);
> -	__llist_add_batch(head, tail, &sr.srs_wait);
> +	return start_new_poll;
>  }
>  
>  static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> @@ -1493,6 +1684,7 @@ static noinline_for_stack bool rcu_gp_init(void)
>  	unsigned long mask;
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp = rcu_get_root();
> +	bool start_new_poll;
>  
>  	WRITE_ONCE(rcu_state.gp_activity, jiffies);
>  	raw_spin_lock_irq_rcu_node(rnp);
> @@ -1517,11 +1709,15 @@ static noinline_for_stack bool rcu_gp_init(void)
>  	/* Record GP times before starting GP, hence rcu_seq_start(). */
>  	rcu_seq_start(&rcu_state.gp_seq);
>  	ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> -	rcu_sr_normal_gp_init();
> +	start_new_poll = rcu_sr_normal_gp_init();
>  	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
>  	rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
>  	raw_spin_unlock_irq_rcu_node(rnp);
>  
> +	// New poll request after rnp unlock
> +	if (start_new_poll)
> +		(void) start_poll_synchronize_rcu();

You lost me on this one.  Anything that got moved to the wait list
should be handled by the current grace period, right?  Or is the
problem that rcu_sr_normal_gp_init() is being invoked after the call
to rcu_seq_start()?  If that is the case, could it be moved ahead so
that we don't need the extra grace period?

Or am I missing something subtle here?

> +
>  	/*
>  	 * Apply per-leaf buffered online and offline operations to
>  	 * the rcu_node tree. Note that this new grace period need not
> -- 
> 2.39.2
> 

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