From: Neeraj Upadhyay <[email protected]> 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. Co-developed-by: Uladzislau Rezki (Sony) <[email protected]> Signed-off-by: Neeraj Upadhyay <[email protected]> Signed-off-by: Uladzislau Rezki (Sony) <[email protected]> --- kernel/rcu/tree.c | 271 +++++++++++++++++++++++++++++++++++++++------- kernel/rcu/tree.h | 13 +++ 2 files changed, 244 insertions(+), 40 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 7d2ed89efcb3..88a47a6a658e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1423,23 +1423,157 @@ static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap) } /* - * 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. */ +static bool rcu_sr_is_wait_head(struct llist_node *node) +{ + return &(rcu_state.srs_wait_nodes)[0].node <= node && + node <= &(rcu_state.srs_wait_nodes)[SR_NORMAL_GP_WAIT_HEAD_MAX - 1].node; +} - /* - * 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; -} sr; +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 = &(rcu_state.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; @@ -1462,14 +1596,44 @@ 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(&rcu_state.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); @@ -1478,48 +1642,61 @@ 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 = rcu_state.srs_wait_tail; + if (wait_tail == NULL) return; - tail = READ_ONCE(sr.srs_wait_tail); - head = __llist_del_all(&sr.srs_wait); + rcu_state.srs_wait_tail = NULL; + ASSERT_EXCLUSIVE_WRITER(rcu_state.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(&rcu_state.srs_done_tail, wait_tail); + ASSERT_EXCLUSIVE_WRITER(rcu_state.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(rcu_state.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, &rcu_state.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(rcu_state.srs_wait_tail != NULL); + rcu_state.srs_wait_tail = wait_head; + ASSERT_EXCLUSIVE_WRITER(rcu_state.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) { - llist_add((struct llist_node *) &rs->head, &sr.srs_next); + llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next); } /* @@ -1532,6 +1709,7 @@ 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); @@ -1556,11 +1734,24 @@ 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); + /* + * The "start_new_poll" is set to true, only when this GP is not able + * to handle anything and there are outstanding users. It happens when + * the rcu_sr_normal_gp_init() function was not able to insert a dummy + * separator to the llist, because there were no left any dummy-nodes. + * + * Number of dummy-nodes is fixed, it could be that we are run out of + * them, if so we start a new pool request to repeat a try. It is rare + * and it means that a system is doing a slow processing of callbacks. + */ + if (start_new_poll) + (void) start_poll_synchronize_rcu(); + /* * Apply per-leaf buffered online and offline operations to * the rcu_node tree. Note that this new grace period need not diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index e9821a8422db..4c35d7d37647 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -316,6 +316,13 @@ do { \ __set_current_state(TASK_RUNNING); \ } while (0) +#define SR_NORMAL_GP_WAIT_HEAD_MAX 5 + +struct sr_wait_node { + atomic_t inuse; + struct llist_node node; +}; + /* * RCU global state, including node hierarchy. This hierarchy is * represented in "heap" form in a dense array. The root (first level) @@ -401,6 +408,12 @@ struct rcu_state { /* Synchronize offline with */ /* GP pre-initialization. */ int nocb_is_setup; /* nocb is setup from boot */ + + /* synchronize_rcu() part. */ + struct llist_head srs_next; /* request a GP users. */ + 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]; }; /* Values for rcu_state structure's gp_flags field. */ -- 2.39.2
