From patchwork Tue Nov 28 08:00:30 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Uladzislau Rezki X-Patchwork-Id: 13470583 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="AXXnuMTy" Received: from mail-lf1-x12e.google.com (mail-lf1-x12e.google.com [IPv6:2a00:1450:4864:20::12e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D799998; Tue, 28 Nov 2023 00:00:41 -0800 (PST) Received: by mail-lf1-x12e.google.com with SMTP id 2adb3069b0e04-50a6ff9881fso7791299e87.1; Tue, 28 Nov 2023 00:00:41 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1701158440; x=1701763240; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=yCjEWa8Q2f6t37ckyMp7lNr3JFQtn/U/ClZD7LTuOUg=; b=AXXnuMTyb0N7xumjFXww93ZjtMgbWMphmbg+ENogVVKejEszhJFXAI3FS789kpqic8 bsZHnNz7E9pg/Jk0q0qzXtrY7msfgnPGje2x0WWMimxbawmjsiyaMdPgj7jwcbazd2C/ UGMym+8lxL5Gh+ZCReU6+Qin3c1ikueQTpZFrptmYBGRm7MZlUtyO7JIhq72hoPIOQos HVp5txnWjlFqUgY5qo5gK50ioAYftAHCLU8XNyYDVs7q9BAunqp5iCjhTYjJhcWDBitV 5yHihsM4bREmcpiAO+TmnZhJF8vBAaBAn9pEyga5+EnkC/XDagAVGpM3k8TI0CIOT6YM 3akg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1701158440; x=1701763240; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=yCjEWa8Q2f6t37ckyMp7lNr3JFQtn/U/ClZD7LTuOUg=; b=uqrCQ1ONq0W2lDQ5D5u1cIA8mA6szC9EHKZessk5jpQN+J8pulxwSEJD5YbpyfD9Tm izY2vrTUAduh1myrABxUUAZuzs+60Ne/JL19UsVlOq0YMPkbze3NMULPsPHqqQJTd7Xh DPB4DcAOMKGwIsUBWhd3vmPEztS8CHxQTWCvgl/+895F5S3aI8ROpIr174ZYohz/deHK 2oliRTIV6IZc8P3Ua5TnowlPsH4LKaoDogTTev4Yg1c2ReVfaMFHrs037T1kNfkF516B kmp1M0NOWwT1XvhyiCiT/prQZTnkkkzvrVcJoDQic4K87gQms+4uizTEkwfOk6c0OYSu mZ0g== X-Gm-Message-State: AOJu0YxvSRUCxialhGFYDTvvHL6FbQAdO7Qi6Lrlhf7XqVaw9OhnCYuN O0x0z8D5cAXNPU+XlqzFS4Ap+JdPM6Q= X-Google-Smtp-Source: AGHT+IEuM/s74Np5OlgdJ9Nq72IMiFoGnCWNA+XDMCq04zLOvZLRTZaS4obMnKQ5drpdoqdD41HdDQ== X-Received: by 2002:a19:910e:0:b0:507:a04c:1bcf with SMTP id t14-20020a19910e000000b00507a04c1bcfmr7806324lfd.58.1701158439522; Tue, 28 Nov 2023 00:00:39 -0800 (PST) Received: from pc638.lan ([155.137.26.201]) by smtp.gmail.com with ESMTPSA id o16-20020ac24bd0000000b004fe202a5c7csm1765501lfq.135.2023.11.28.00.00.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Nov 2023 00:00:39 -0800 (PST) From: "Uladzislau Rezki (Sony)" To: "Paul E . McKenney" Cc: RCU , Neeraj upadhyay , Boqun Feng , Hillf Danton , Joel Fernandes , LKML , Uladzislau Rezki , Oleksiy Avramchenko , Frederic Weisbecker Subject: [PATCH v3 4/7] rcu: Improve handling of synchronize_rcu() users Date: Tue, 28 Nov 2023 09:00:30 +0100 Message-Id: <20231128080033.288050-5-urezki@gmail.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20231128080033.288050-1-urezki@gmail.com> References: <20231128080033.288050-1-urezki@gmail.com> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Neeraj Upadhyay 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) Signed-off-by: Neeraj Upadhyay --- 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(); + /* * Apply per-leaf buffered online and offline operations to * the rcu_node tree. Note that this new grace period need not