/* * Copyright (c) 2012 Adam Hraska * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * * - Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * - Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * - The name of the author may not be used to endorse or promote products * derived from this software without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ /** @addtogroup sync * @{ */ /** * @file * @brief Preemptible read-copy update. Usable from interrupt handlers. */ #include #include #include #include #include #include #include #include #include #include #include #include /* * Number of milliseconds to give to preexisting readers to finish * when non-expedited grace period detection is in progress. */ #define DETECT_SLEEP_MS 10 /* * Max number of pending callbacks in the local cpu's queue before * aggressively expediting the current grace period */ #define EXPEDITE_THRESHOLD 2000 /* * Max number of callbacks to execute in one go with preemption * enabled. If there are more callbacks to be executed they will * be run with preemption disabled in order to prolong reclaimer's * time slice and give it a chance to catch up with callback producers. */ #define CRITICAL_THRESHOLD 30000 /* Half the number of values a uint32 can hold. */ #define UINT32_MAX_HALF 2147483648U /** Global RCU data. */ typedef struct rcu_data { /** Detector uses so signal reclaimers that a grace period ended. */ condvar_t gp_ended; /** Reclaimers notify the detector when they request more grace periods.*/ condvar_t req_gp_changed; /** Reclaimers use to notify the detector to accelerate GP detection. */ condvar_t expedite_now; /** * The detector waits on this semaphore for any readers delaying the GP. * * Each of the cpus with readers that are delaying the current GP * must up() this sema once they reach a quiescent state. If there * are any readers in cur_preempted (ie preempted preexisting) and * they are already delaying GP detection, the last to unlock its * reader section must up() this sema once. */ semaphore_t remaining_readers; /** Protects the 4 fields below. */ SPINLOCK_DECLARE(gp_lock); /** Number of grace period ends the detector was requested to announce. */ size_t req_gp_end_cnt; /** Number of consecutive grace periods to detect quickly and aggressively.*/ size_t req_expedited_cnt; /** * The current grace period number. Increases monotonically. * Lock gp_lock or preempt_lock to get a current value. */ rcu_gp_t cur_gp; /** * The number of the most recently completed grace period. * At most one behind cur_gp. If equal to cur_gp, a grace * period detection is not in progress and the detector * is idle. */ rcu_gp_t completed_gp; /** Protects the following 3 fields. */ IRQ_SPINLOCK_DECLARE(preempt_lock); /** Preexisting readers that have been preempted. */ list_t cur_preempted; /** Reader that have been preempted and might delay the next grace period.*/ list_t next_preempted; /** * The detector is waiting for the last preempted reader * in cur_preempted to announce that it exited its reader * section by up()ing remaining_readers. */ bool preempt_blocking_det; /** * Number of cpus with readers that are delaying the current GP. * They will up() remaining_readers. */ atomic_t delaying_cpu_cnt; /** Interruptible attached detector thread pointer. */ thread_t *detector_thr; /* Some statistics. */ size_t stat_expedited_cnt; size_t stat_delayed_cnt; size_t stat_preempt_blocking_cnt; /* Does not contain self/local calls. */ size_t stat_smp_call_cnt; } rcu_data_t; static rcu_data_t rcu; static void start_detector(void); static void start_reclaimers(void); static void rcu_read_unlock_impl(size_t *pnesting_cnt); static void synch_complete(rcu_item_t *rcu_item); static void check_qs(void); static void record_qs(void); static void signal_read_unlock(void); static bool arriving_cbs_empty(void); static bool next_cbs_empty(void); static bool cur_cbs_empty(void); static bool all_cbs_empty(void); static void reclaimer(void *arg); static bool wait_for_pending_cbs(void); static bool advance_cbs(void); static void exec_completed_cbs(rcu_gp_t last_completed_gp); static void exec_cbs(rcu_item_t **phead); static void req_detection(size_t req_cnt); static bool wait_for_cur_cbs_gp_end(bool expedite, rcu_gp_t *last_completed_gp); static void upd_missed_gp_in_wait(rcu_gp_t completed_gp); static bool cv_wait_for_gp(rcu_gp_t wait_on_gp); static void detector(void *); static bool wait_for_detect_req(void); static void start_new_gp(void); static void end_cur_gp(void); static bool wait_for_readers(void); static void rm_quiescent_cpus(cpu_mask_t *cpu_mask); static bool gp_sleep(void); static void interrupt_delaying_cpus(cpu_mask_t *cpu_mask); static void sample_local_cpu(void *); static bool wait_for_delaying_cpus(void); static bool wait_for_preempt_reader(void); static void upd_max_cbs_in_slice(void); /** Initializes global RCU structures. */ void rcu_init(void) { condvar_initialize(&rcu.gp_ended); condvar_initialize(&rcu.req_gp_changed); condvar_initialize(&rcu.expedite_now); semaphore_initialize(&rcu.remaining_readers, 0); spinlock_initialize(&rcu.gp_lock, "rcu.gp_lock"); rcu.req_gp_end_cnt = 0; rcu.req_expedited_cnt = 0; rcu.cur_gp = 0; rcu.completed_gp = 0; irq_spinlock_initialize(&rcu.preempt_lock, "rcu.preempt_lock"); list_initialize(&rcu.cur_preempted); list_initialize(&rcu.next_preempted); rcu.preempt_blocking_det = false; atomic_set(&rcu.delaying_cpu_cnt, 0); rcu.detector_thr = 0; rcu.stat_expedited_cnt = 0; rcu.stat_delayed_cnt = 0; rcu.stat_preempt_blocking_cnt = 0; rcu.stat_smp_call_cnt = 0; } /** Initializes per-CPU RCU data. If on the boot cpu inits global data too.*/ void rcu_cpu_init(void) { if (config.cpu_active == 1) { rcu_init(); } CPU->rcu.last_seen_gp = 0; CPU->rcu.pnesting_cnt = &CPU->rcu.tmp_nesting_cnt; CPU->rcu.tmp_nesting_cnt = 0; CPU->rcu.cur_cbs = 0; CPU->rcu.cur_cbs_cnt = 0; CPU->rcu.next_cbs = 0; CPU->rcu.next_cbs_cnt = 0; CPU->rcu.arriving_cbs = 0; CPU->rcu.parriving_cbs_tail = &CPU->rcu.arriving_cbs; CPU->rcu.arriving_cbs_cnt = 0; CPU->rcu.cur_cbs_gp = 0; CPU->rcu.next_cbs_gp = 0; CPU->rcu.is_delaying_gp = false; semaphore_initialize(&CPU->rcu.arrived_flag, 0); /* BSP creates reclaimer threads before AP's rcu_cpu_init() runs. */ if (config.cpu_active == 1) CPU->rcu.reclaimer_thr = 0; CPU->rcu.stat_max_cbs = 0; CPU->rcu.stat_avg_cbs = 0; CPU->rcu.stat_missed_gps = 0; CPU->rcu.stat_missed_gp_in_wait = 0; CPU->rcu.stat_max_slice_cbs = 0; CPU->rcu.last_arriving_cnt = 0; } /** Completes RCU init. Creates and runs the detector and reclaimer threads.*/ void rcu_kinit_init(void) { start_detector(); start_reclaimers(); } /** Initializes any per-thread RCU structures. */ void rcu_thread_init(thread_t *thread) { thread->rcu.nesting_cnt = 0; thread->rcu.was_preempted = false; link_initialize(&thread->rcu.preempt_link); } /** Called from scheduler() when exiting the current thread. * * Preemption or interrupts are disabled and the scheduler() already * switched away from the current thread, calling rcu_after_thread_ran(). */ void rcu_thread_exiting(void) { ASSERT(THREAD != 0); ASSERT(THREAD->state == Exiting); ASSERT(PREEMPTION_DISABLED || interrupts_disabled()); /* * The scheduler() must have already switched to a temporary * nesting counter for interrupt handlers (we could be idle) * so that interrupt handlers do not modify the exiting thread's * reader section nesting count while we examine/process it. */ ASSERT(&CPU->rcu.tmp_nesting_cnt == CPU->rcu.pnesting_cnt); /* * The thread forgot to exit its reader critical secion. * It is a bug, but rather than letting the entire system lock up * forcefully leave the reader section. The thread is not holding * any references anyway since it is exiting so it is safe. */ if (0 < THREAD->rcu.nesting_cnt) { THREAD->rcu.nesting_cnt = 1; rcu_read_unlock_impl(&THREAD->rcu.nesting_cnt); } } /** Cleans up global RCU resources and stops dispatching callbacks. * * Call when shutting down the kernel. Outstanding callbacks will * not be processed. Instead they will linger forever. */ void rcu_stop(void) { /* todo: stop accepting new callbacks instead of just letting them linger?*/ /* Stop and wait for reclaimers. */ for (unsigned int cpu_id = 0; cpu_id < config.cpu_active; ++cpu_id) { ASSERT(cpus[cpu_id].rcu.reclaimer_thr != 0); if (cpus[cpu_id].rcu.reclaimer_thr) { thread_interrupt(cpus[cpu_id].rcu.reclaimer_thr); thread_join(cpus[cpu_id].rcu.reclaimer_thr); thread_detach(cpus[cpu_id].rcu.reclaimer_thr); cpus[cpu_id].rcu.reclaimer_thr = 0; } } /* Stop the detector and wait. */ if (rcu.detector_thr) { thread_interrupt(rcu.detector_thr); thread_join(rcu.detector_thr); thread_detach(rcu.detector_thr); rcu.detector_thr = 0; } } /** Starts the detector thread. */ static void start_detector(void) { rcu.detector_thr = thread_create(detector, 0, TASK, THREAD_FLAG_NONE, "rcu-det"); if (!rcu.detector_thr) panic("Failed to create RCU detector thread."); thread_ready(rcu.detector_thr); } /** Creates and runs cpu-bound reclaimer threads. */ static void start_reclaimers(void) { for (unsigned int cpu_id = 0; cpu_id < config.cpu_count; ++cpu_id) { char name[THREAD_NAME_BUFLEN] = {0}; snprintf(name, THREAD_NAME_BUFLEN - 1, "rcu-rec/%u", cpu_id); cpus[cpu_id].rcu.reclaimer_thr = thread_create(reclaimer, 0, TASK, THREAD_FLAG_NONE, name); if (!cpus[cpu_id].rcu.reclaimer_thr) panic("Failed to create RCU reclaimer thread on cpu%u.", cpu_id); thread_wire(cpus[cpu_id].rcu.reclaimer_thr, &cpus[cpu_id]); thread_ready(cpus[cpu_id].rcu.reclaimer_thr); } } /** Returns the number of elapsed grace periods since boot. */ uint64_t rcu_completed_gps(void) { spinlock_lock(&rcu.gp_lock); uint64_t completed = rcu.completed_gp; spinlock_unlock(&rcu.gp_lock); return completed; } /** Delimits the start of an RCU reader critical section. * * Reader sections may be nested and are preemptable. You must not * however block/sleep within reader sections. */ void rcu_read_lock(void) { ASSERT(CPU); preemption_disable(); check_qs(); ++(*CPU->rcu.pnesting_cnt); preemption_enable(); } /** Delimits the end of an RCU reader critical section. */ void rcu_read_unlock(void) { ASSERT(CPU); preemption_disable(); rcu_read_unlock_impl(CPU->rcu.pnesting_cnt); preemption_enable(); } /** Unlocks the local reader section using the given nesting count. * * Preemption or interrupts must be disabled. * * @param pnesting_cnt Either &CPU->rcu.tmp_nesting_cnt or * THREAD->rcu.nesting_cnt. */ static void rcu_read_unlock_impl(size_t *pnesting_cnt) { ASSERT(PREEMPTION_DISABLED || interrupts_disabled()); if (0 == --(*pnesting_cnt)) { record_qs(); /* * The thread was preempted while in a critical section or * the detector is eagerly waiting for this cpu's reader * to finish. * * Note that THREAD may be 0 in scheduler() and not just during boot. */ if ((THREAD && THREAD->rcu.was_preempted) || CPU->rcu.is_delaying_gp) { /* Rechecks with disabled interrupts. */ signal_read_unlock(); } } } /** Records a QS if not in a reader critical section. */ static void check_qs(void) { ASSERT(PREEMPTION_DISABLED || interrupts_disabled()); if (0 == *CPU->rcu.pnesting_cnt) record_qs(); } /** Unconditionally records a quiescent state for the local cpu. */ static void record_qs(void) { ASSERT(PREEMPTION_DISABLED || interrupts_disabled()); /* * A new GP was started since the last time we passed a QS. * Notify the detector we have reached a new QS. */ if (CPU->rcu.last_seen_gp != rcu.cur_gp) { rcu_gp_t cur_gp = ACCESS_ONCE(rcu.cur_gp); /* * Contain memory accesses within a reader critical section. * If we are in rcu_lock() it also makes changes prior to the * start of the GP visible in the reader section. */ memory_barrier(); /* * Acknowledge we passed a QS since the beginning of rcu.cur_gp. * Cache coherency will lazily transport the value to the * detector while it sleeps in gp_sleep(). * * Note that there is a theoretical possibility that we * overwrite a more recent/greater last_seen_gp here with * an older/smaller value. If this cpu is interrupted here * while in rcu_lock() reader sections in the interrupt handler * will update last_seen_gp to the same value as is currently * in local cur_gp. However, if the cpu continues processing * interrupts and the detector starts a new GP immediately, * local interrupt handlers may update last_seen_gp again (ie * properly ack the new GP) with a value greater than local cur_gp. * Resetting last_seen_gp to a previous value here is however * benign and we only have to remember that this reader may end up * in cur_preempted even after the GP ends. That is why we * append next_preempted to cur_preempted rather than overwriting * it as if cur_preempted were empty. */ CPU->rcu.last_seen_gp = cur_gp; } } /** If necessary, signals the detector that we exited a reader section. */ static void signal_read_unlock(void) { ASSERT(PREEMPTION_DISABLED || interrupts_disabled()); /* * We have to disable interrupts in order to make checking * and resetting was_preempted and is_delaying_gp atomic * with respect to local interrupt handlers. Otherwise * an interrupt could beat us to calling semaphore_up() * before we reset the appropriate flag. */ ipl_t ipl = interrupts_disable(); /* * If the detector is eagerly waiting for this cpu's reader to unlock, * notify it that the reader did so. */ if (CPU->rcu.is_delaying_gp) { CPU->rcu.is_delaying_gp = false; semaphore_up(&rcu.remaining_readers); } /* * This reader was preempted while in a reader section. * We might be holding up the current GP. Notify the * detector if so. */ if (THREAD && THREAD->rcu.was_preempted) { ASSERT(link_used(&THREAD->rcu.preempt_link)); THREAD->rcu.was_preempted = false; irq_spinlock_lock(&rcu.preempt_lock, false); bool prev_empty = list_empty(&rcu.cur_preempted); list_remove(&THREAD->rcu.preempt_link); bool now_empty = list_empty(&rcu.cur_preempted); /* This was the last reader in cur_preempted. */ bool last_removed = now_empty && !prev_empty; /* * Preempted readers are blocking the detector and * this was the last reader blocking the current GP. */ if (last_removed && rcu.preempt_blocking_det) { rcu.preempt_blocking_det = false; semaphore_up(&rcu.remaining_readers); } irq_spinlock_unlock(&rcu.preempt_lock, false); } interrupts_restore(ipl); } typedef struct synch_item { waitq_t wq; rcu_item_t rcu_item; } synch_item_t; /** Blocks until all preexisting readers exit their critical sections. */ void rcu_synchronize(void) { /* Calling from a reader section will deadlock. */ ASSERT(THREAD == 0 || 0 == THREAD->rcu.nesting_cnt); synch_item_t completion; waitq_initialize(&completion.wq); rcu_call(&completion.rcu_item, synch_complete); waitq_sleep(&completion.wq); waitq_complete_wakeup(&completion.wq); } /** rcu_synchronize's callback. */ static void synch_complete(rcu_item_t *rcu_item) { synch_item_t *completion = member_to_inst(rcu_item, synch_item_t, rcu_item); ASSERT(completion); waitq_wakeup(&completion->wq, WAKEUP_FIRST); } /** Adds a callback to invoke after all preexisting readers finish. * * May be called from within interrupt handlers or RCU reader sections. * * @param rcu_item Used by RCU to track the call. Must remain * until the user callback function is entered. * @param func User callback function that will be invoked once a full * grace period elapsed, ie at a time when all preexisting * readers have finished. The callback should be short and must * not block. If you must sleep, enqueue your work in the system * work queue from the callback (ie workq_global_enqueue()). */ void rcu_call(rcu_item_t *rcu_item, rcu_func_t func) { _rcu_call(false, rcu_item, func); } /** rcu_call() implementation. See rcu_call() for comments. */ void _rcu_call(bool expedite, rcu_item_t *rcu_item, rcu_func_t func) { ASSERT(rcu_item); rcu_item->func = func; rcu_item->next = 0; preemption_disable(); ipl_t ipl = interrupts_disable(); *CPU->rcu.parriving_cbs_tail = rcu_item; CPU->rcu.parriving_cbs_tail = &rcu_item->next; size_t cnt = ++CPU->rcu.arriving_cbs_cnt; interrupts_restore(ipl); if (expedite) { CPU->rcu.expedite_arriving = true; } /* Added first callback - notify the reclaimer. */ if (cnt == 1 && !semaphore_count_get(&CPU->rcu.arrived_flag)) { semaphore_up(&CPU->rcu.arrived_flag); } preemption_enable(); } static bool cur_cbs_empty(void) { ASSERT(THREAD && THREAD->wired); return 0 == CPU->rcu.cur_cbs; } static bool next_cbs_empty(void) { ASSERT(THREAD && THREAD->wired); return 0 == CPU->rcu.next_cbs; } /** Disable interrupts to get an up-to-date result. */ static bool arriving_cbs_empty(void) { ASSERT(THREAD && THREAD->wired); /* * Accessing with interrupts enabled may at worst lead to * a false negative if we race with a local interrupt handler. */ return 0 == CPU->rcu.arriving_cbs; } static bool all_cbs_empty(void) { return cur_cbs_empty() && next_cbs_empty() && arriving_cbs_empty(); } /** Reclaimer thread dispatches locally queued callbacks once a GP ends. */ static void reclaimer(void *arg) { ASSERT(THREAD && THREAD->wired); ASSERT(THREAD == CPU->rcu.reclaimer_thr); rcu_gp_t last_compl_gp = 0; bool ok = true; while (ok && wait_for_pending_cbs()) { ASSERT(CPU->rcu.reclaimer_thr == THREAD); exec_completed_cbs(last_compl_gp); bool expedite = advance_cbs(); ok = wait_for_cur_cbs_gp_end(expedite, &last_compl_gp); } } /** Waits until there are callbacks waiting to be dispatched. */ static bool wait_for_pending_cbs(void) { if (!all_cbs_empty()) return true; bool ok = true; while (arriving_cbs_empty() && ok) { ok = semaphore_down_interruptable(&CPU->rcu.arrived_flag); } return ok; } static void upd_stat_missed_gp(rcu_gp_t compl) { if (CPU->rcu.cur_cbs_gp < compl) { CPU->rcu.stat_missed_gps += (size_t)(compl - CPU->rcu.cur_cbs_gp); } } /** Executes all callbacks for the given completed grace period. */ static void exec_completed_cbs(rcu_gp_t last_completed_gp) { upd_stat_missed_gp(last_completed_gp); /* Both next_cbs and cur_cbs GP elapsed. */ if (CPU->rcu.next_cbs_gp <= last_completed_gp) { ASSERT(CPU->rcu.cur_cbs_gp <= CPU->rcu.next_cbs_gp); size_t exec_cnt = CPU->rcu.cur_cbs_cnt + CPU->rcu.next_cbs_cnt; if (exec_cnt < CRITICAL_THRESHOLD) { exec_cbs(&CPU->rcu.cur_cbs); exec_cbs(&CPU->rcu.next_cbs); } else { /* * Getting overwhelmed with too many callbacks to run. * Disable preemption in order to prolong our time slice * and catch up with updaters posting new callbacks. */ preemption_disable(); exec_cbs(&CPU->rcu.cur_cbs); exec_cbs(&CPU->rcu.next_cbs); preemption_enable(); } CPU->rcu.cur_cbs_cnt = 0; CPU->rcu.next_cbs_cnt = 0; } else if (CPU->rcu.cur_cbs_gp <= last_completed_gp) { if (CPU->rcu.cur_cbs_cnt < CRITICAL_THRESHOLD) { exec_cbs(&CPU->rcu.cur_cbs); } else { /* * Getting overwhelmed with too many callbacks to run. * Disable preemption in order to prolong our time slice * and catch up with updaters posting new callbacks. */ preemption_disable(); exec_cbs(&CPU->rcu.cur_cbs); preemption_enable(); } CPU->rcu.cur_cbs_cnt = 0; } } /** Executes callbacks in the single-linked list. The list is left empty. */ static void exec_cbs(rcu_item_t **phead) { rcu_item_t *rcu_item = *phead; while (rcu_item) { /* func() may free rcu_item. Get a local copy. */ rcu_item_t *next = rcu_item->next; rcu_func_t func = rcu_item->func; func(rcu_item); rcu_item = next; } *phead = 0; } static void upd_stat_cb_cnts(size_t arriving_cnt) { CPU->rcu.stat_max_cbs = max(arriving_cnt, CPU->rcu.stat_max_cbs); if (0 < arriving_cnt) { CPU->rcu.stat_avg_cbs = (99 * CPU->rcu.stat_avg_cbs + 1 * arriving_cnt) / 100; } } /** Prepares another batch of callbacks to dispatch at the nest grace period. * * @return True if the next batch of callbacks must be expedited quickly. */ static bool advance_cbs(void) { /* Move next_cbs to cur_cbs. */ CPU->rcu.cur_cbs = CPU->rcu.next_cbs; CPU->rcu.cur_cbs_cnt = CPU->rcu.next_cbs_cnt; CPU->rcu.cur_cbs_gp = CPU->rcu.next_cbs_gp; /* Move arriving_cbs to next_cbs. Empties arriving_cbs. */ ipl_t ipl = interrupts_disable(); /* * Too many callbacks queued. Better speed up the detection * or risk exhausting all system memory. */ bool expedite = (EXPEDITE_THRESHOLD < CPU->rcu.arriving_cbs_cnt) || CPU->rcu.expedite_arriving; CPU->rcu.expedite_arriving = false; CPU->rcu.next_cbs = CPU->rcu.arriving_cbs; CPU->rcu.next_cbs_cnt = CPU->rcu.arriving_cbs_cnt; CPU->rcu.arriving_cbs = 0; CPU->rcu.parriving_cbs_tail = &CPU->rcu.arriving_cbs; CPU->rcu.arriving_cbs_cnt = 0; interrupts_restore(ipl); /* Update statistics of arrived callbacks. */ upd_stat_cb_cnts(CPU->rcu.next_cbs_cnt); /* * Make changes prior to queuing next_cbs visible to readers. * See comment in wait_for_readers(). */ memory_barrier(); /* MB A, B */ /* At the end of next_cbs_gp, exec next_cbs. Determine what GP that is. */ if (!next_cbs_empty()) { spinlock_lock(&rcu.gp_lock); /* Exec next_cbs at the end of the next GP. */ CPU->rcu.next_cbs_gp = rcu.cur_gp + 1; /* * There are no callbacks to invoke before next_cbs. Instruct * wait_for_cur_cbs_gp() to notify us of the nearest GP end. * That could be sooner than next_cbs_gp (if the current GP * had not yet completed), so we'll create a shorter batch * of callbacks next time around. */ if (cur_cbs_empty()) { CPU->rcu.cur_cbs_gp = rcu.completed_gp + 1; } spinlock_unlock(&rcu.gp_lock); } else { CPU->rcu.next_cbs_gp = CPU->rcu.cur_cbs_gp; } ASSERT(CPU->rcu.cur_cbs_gp <= CPU->rcu.next_cbs_gp); return expedite; } /** Waits for the grace period associated with callbacks cub_cbs to elapse. * * @param expedite Instructs the detector to aggressively speed up grace * period detection without any delay. * @param completed_gp Returns the most recent completed grace period * number. * @return false if the thread was interrupted and should stop. */ static bool wait_for_cur_cbs_gp_end(bool expedite, rcu_gp_t *completed_gp) { /* * Use a possibly outdated version of completed_gp to bypass checking * with the lock. * * Note that loading and storing rcu.completed_gp is not atomic * (it is 64bit wide). Reading a clobbered value that is less than * rcu.completed_gp is harmless - we'll recheck with a lock. The * only way to read a clobbered value that is greater than the actual * value is if the detector increases the higher-order word first and * then decreases the lower-order word (or we see stores in that order), * eg when incrementing from 2^32 - 1 to 2^32. The loaded value * suddenly jumps by 2^32. It would take hours for such an increase * to occur so it is safe to discard the value. We allow increases * of up to half the maximum to generously accommodate for loading an * outdated lower word. */ rcu_gp_t compl_gp = ACCESS_ONCE(rcu.completed_gp); if (CPU->rcu.cur_cbs_gp <= compl_gp && compl_gp <= CPU->rcu.cur_cbs_gp + UINT32_MAX_HALF) { *completed_gp = compl_gp; return true; } spinlock_lock(&rcu.gp_lock); if (CPU->rcu.cur_cbs_gp <= rcu.completed_gp) { *completed_gp = rcu.completed_gp; spinlock_unlock(&rcu.gp_lock); return true; } ASSERT(CPU->rcu.cur_cbs_gp <= CPU->rcu.next_cbs_gp); ASSERT(rcu.cur_gp <= CPU->rcu.cur_cbs_gp); /* * Notify the detector of how many GP ends we intend to wait for, so * it can avoid going to sleep unnecessarily. Optimistically assume * new callbacks will arrive while we're waiting; hence +1. */ size_t remaining_gp_ends = (size_t) (CPU->rcu.next_cbs_gp - rcu.cur_gp); req_detection(remaining_gp_ends + (arriving_cbs_empty() ? 0 : 1)); /* * Ask the detector to speed up GP detection if there are too many * pending callbacks and other reclaimers have not already done so. */ if (expedite) { if(0 == rcu.req_expedited_cnt) condvar_signal(&rcu.expedite_now); /* * Expedite only cub_cbs. If there really is a surge of callbacks * the arriving batch will expedite the GP for the huge number * of callbacks currently in next_cbs */ rcu.req_expedited_cnt = 1; } /* Wait for cur_cbs_gp to end. */ bool interrupted = cv_wait_for_gp(CPU->rcu.cur_cbs_gp); *completed_gp = rcu.completed_gp; spinlock_unlock(&rcu.gp_lock); upd_missed_gp_in_wait(*completed_gp); return !interrupted; } static void upd_missed_gp_in_wait(rcu_gp_t completed_gp) { ASSERT(CPU->rcu.cur_cbs_gp <= completed_gp); size_t delta = (size_t)(completed_gp - CPU->rcu.cur_cbs_gp); CPU->rcu.stat_missed_gp_in_wait += delta; } /** Requests the detector to detect at least req_cnt consecutive grace periods.*/ static void req_detection(size_t req_cnt) { if (rcu.req_gp_end_cnt < req_cnt) { bool detector_idle = (0 == rcu.req_gp_end_cnt); rcu.req_gp_end_cnt = req_cnt; if (detector_idle) { ASSERT(rcu.cur_gp == rcu.completed_gp); condvar_signal(&rcu.req_gp_changed); } } } /** Waits for an announcement of the end of the grace period wait_on_gp. */ static bool cv_wait_for_gp(rcu_gp_t wait_on_gp) { ASSERT(spinlock_locked(&rcu.gp_lock)); bool interrupted = false; /* Wait until wait_on_gp ends. */ while (rcu.completed_gp < wait_on_gp && !interrupted) { int ret = _condvar_wait_timeout_spinlock(&rcu.gp_ended, &rcu.gp_lock, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_INTERRUPTIBLE); interrupted = (ret == ESYNCH_INTERRUPTED); } ASSERT(wait_on_gp <= rcu.completed_gp); return interrupted; } /** The detector thread detects and notifies reclaimers of grace period ends. */ static void detector(void *arg) { spinlock_lock(&rcu.gp_lock); while (wait_for_detect_req()) { /* * Announce new GP started. Readers start lazily acknowledging that * they passed a QS. */ start_new_gp(); spinlock_unlock(&rcu.gp_lock); if (!wait_for_readers()) goto unlocked_out; spinlock_lock(&rcu.gp_lock); /* Notify reclaimers that they may now invoke queued callbacks. */ end_cur_gp(); } spinlock_unlock(&rcu.gp_lock); unlocked_out: return; } /** Waits for a request from a reclaimer thread to detect a grace period. */ static bool wait_for_detect_req(void) { ASSERT(spinlock_locked(&rcu.gp_lock)); bool interrupted = false; while (0 == rcu.req_gp_end_cnt && !interrupted) { int ret = _condvar_wait_timeout_spinlock(&rcu.req_gp_changed, &rcu.gp_lock, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_INTERRUPTIBLE); interrupted = (ret == ESYNCH_INTERRUPTED); } return !interrupted; } /** Announces the start of a new grace period for preexisting readers to ack. */ static void start_new_gp(void) { ASSERT(spinlock_locked(&rcu.gp_lock)); irq_spinlock_lock(&rcu.preempt_lock, true); /* Start a new GP. Announce to readers that a quiescent state is needed. */ ++rcu.cur_gp; /* * Readers preempted before the start of this GP (next_preempted) * are preexisting readers now that a GP started and will hold up * the current GP until they exit their reader sections. * * Preempted readers from the previous GP have finished so * cur_preempted is empty, but see comment in record_qs(). */ list_concat(&rcu.cur_preempted, &rcu.next_preempted); irq_spinlock_unlock(&rcu.preempt_lock, true); } static void end_cur_gp(void) { ASSERT(spinlock_locked(&rcu.gp_lock)); rcu.completed_gp = rcu.cur_gp; --rcu.req_gp_end_cnt; condvar_broadcast(&rcu.gp_ended); } /** Waits for readers that started before the current GP started to finish. */ static bool wait_for_readers(void) { DEFINE_CPU_MASK(reading_cpus); /* All running cpus have potential readers. */ cpu_mask_active(reading_cpus); /* * Ensure the announcement of the start of a new GP (ie up-to-date * cur_gp) propagates to cpus that are just coming out of idle * mode before we sample their idle state flag. * * Cpus guarantee that after they set CPU->idle = true they will not * execute any RCU reader sections without first setting idle to * false and issuing a memory barrier. Therefore, if rm_quiescent_cpus() * later on sees an idle cpu, but the cpu is just exiting its idle mode, * the cpu must not have yet executed its memory barrier (otherwise * it would pair up with this mem barrier and we would see idle == false). * That memory barrier will pair up with the one below and ensure * that a reader on the now-non-idle cpu will see the most current * cur_gp. As a result, such a reader will never attempt to semaphore_up( * pending_readers) during this GP, which allows the detector to * ignore that cpu (the detector thinks it is idle). Moreover, any * changes made by RCU updaters will have propagated to readers * on the previously idle cpu -- again thanks to issuing a memory * barrier after returning from idle mode. * * idle -> non-idle cpu | detector | reclaimer * ------------------------------------------------------ * rcu reader 1 | | rcu_call() * MB X | | * idle = true | | rcu_call() * (no rcu readers allowed ) | | MB A in advance_cbs() * MB Y | (...) | (...) * (no rcu readers allowed) | | MB B in advance_cbs() * idle = false | ++cur_gp | * (no rcu readers allowed) | MB C | * MB Z | signal gp_end | * rcu reader 2 | | exec_cur_cbs() * * * MB Y orders visibility of changes to idle for detector's sake. * * MB Z pairs up with MB C. The cpu making a transition from idle * will see the most current value of cur_gp and will not attempt * to notify the detector even if preempted during this GP. * * MB Z pairs up with MB A from the previous batch. Updaters' changes * are visible to reader 2 even when the detector thinks the cpu is idle * but it is not anymore. * * MB X pairs up with MB B. Late mem accesses of reader 1 are contained * and visible before idling and before any callbacks are executed * by reclaimers. * * In summary, the detector does not know of or wait for reader 2, but * it does not have to since it is a new reader that will not access * data from previous GPs and will see any changes. */ memory_barrier(); /* MB C */ /* * Give readers time to pass through a QS. Also, batch arriving * callbacks in order to amortize detection overhead. */ if (!gp_sleep()) return false; /* Non-intrusively determine which cpus have yet to pass a QS. */ rm_quiescent_cpus(reading_cpus); /* Actively interrupt cpus delaying the current GP and demand a QS. */ interrupt_delaying_cpus(reading_cpus); /* Wait for the interrupted cpus to notify us that they reached a QS. */ if (!wait_for_delaying_cpus()) return false; /* * All cpus recorded a QS or are still idle. Any new readers will be added * to next_preempt if preempted, ie the number of readers in cur_preempted * monotonically descreases. */ /* Wait for the last reader in cur_preempted to notify us it is done. */ if (!wait_for_preempt_reader()) return false; return true; } /** Remove those cpus from the mask that have already passed a quiescent * state since the start of the current grace period. */ static void rm_quiescent_cpus(cpu_mask_t *cpu_mask) { cpu_mask_for_each(*cpu_mask, cpu_id) { /* * The cpu already checked for and passed through a quiescent * state since the beginning of this GP. * * rcu.cur_gp is modified by local detector thread only. * Therefore, it is up-to-date even without a lock. */ bool cpu_acked_gp = (cpus[cpu_id].rcu.last_seen_gp == rcu.cur_gp); /* * Either the cpu is idle or it is exiting away from idle mode * and already sees the most current rcu.cur_gp. See comment * in wait_for_readers(). */ bool cpu_idle = cpus[cpu_id].idle; if (cpu_acked_gp || cpu_idle) { cpu_mask_reset(cpu_mask, cpu_id); } } } /** Sleeps a while if the current grace period is not to be expedited. */ static bool gp_sleep(void) { spinlock_lock(&rcu.gp_lock); int ret = 0; while (0 == rcu.req_expedited_cnt && 0 == ret) { /* minor bug: sleeps for the same duration if woken up spuriously. */ ret = _condvar_wait_timeout_spinlock(&rcu.expedite_now, &rcu.gp_lock, DETECT_SLEEP_MS * 1000, SYNCH_FLAGS_INTERRUPTIBLE); } if (0 < rcu.req_expedited_cnt) { --rcu.req_expedited_cnt; /* Update statistic. */ ++rcu.stat_expedited_cnt; } spinlock_unlock(&rcu.gp_lock); return (ret != ESYNCH_INTERRUPTED); } /** Actively interrupts and checks the offending cpus for quiescent states. */ static void interrupt_delaying_cpus(cpu_mask_t *cpu_mask) { const size_t max_conconcurrent_calls = 16; smp_call_t call[max_conconcurrent_calls]; size_t outstanding_calls = 0; atomic_set(&rcu.delaying_cpu_cnt, 0); cpu_mask_for_each(*cpu_mask, cpu_id) { smp_call_async(cpu_id, sample_local_cpu, 0, &call[outstanding_calls]); ++outstanding_calls; /* Update statistic. */ if (CPU->id != cpu_id) ++rcu.stat_smp_call_cnt; if (outstanding_calls == max_conconcurrent_calls) { for (size_t k = 0; k < outstanding_calls; ++k) { smp_call_wait(&call[k]); } outstanding_calls = 0; } } for (size_t k = 0; k < outstanding_calls; ++k) { smp_call_wait(&call[k]); } } /** Invoked on a cpu delaying grace period detection. * * Induces a quiescent state for the cpu or it instructs remaining * readers to notify the detector once they finish. */ static void sample_local_cpu(void *arg) { ASSERT(interrupts_disabled()); ASSERT(!CPU->rcu.is_delaying_gp); /* Cpu did not pass a quiescent state yet. */ if (CPU->rcu.last_seen_gp != rcu.cur_gp) { /* Interrupted a reader in a reader critical section. */ if (0 < (*CPU->rcu.pnesting_cnt)) { ASSERT(!CPU->idle); /* Note to notify the detector from rcu_read_unlock(). */ CPU->rcu.is_delaying_gp = true; atomic_inc(&rcu.delaying_cpu_cnt); } else { /* * The cpu did not enter any rcu reader sections since * the start of the current GP. Record a quiescent state. * * Or, we interrupted rcu_read_unlock_impl() right before * it recorded a QS. Record a QS for it. The memory barrier * contains the reader section's mem accesses before * updating last_seen_gp. * * Or, we interrupted rcu_read_lock() right after it recorded * a QS for the previous GP but before it got a chance to * increment its nesting count. The memory barrier again * stops the CS code from spilling out of the CS. */ memory_barrier(); CPU->rcu.last_seen_gp = rcu.cur_gp; } } else { /* * This cpu already acknowledged that it had passed through * a quiescent state since the start of cur_gp. */ } /* * smp_call() makes sure any changes propagate back to the caller. * In particular, it makes the most current last_seen_gp visible * to the detector. */ } /** Waits for cpus delaying the current grace period if there are any. */ static bool wait_for_delaying_cpus(void) { int delaying_cpu_cnt = atomic_get(&rcu.delaying_cpu_cnt); for (int i = 0; i < delaying_cpu_cnt; ++i){ if (!semaphore_down_interruptable(&rcu.remaining_readers)) return false; } /* Update statistic. */ rcu.stat_delayed_cnt += delaying_cpu_cnt; return true; } /** Waits for any preempted readers blocking this grace period to finish.*/ static bool wait_for_preempt_reader(void) { irq_spinlock_lock(&rcu.preempt_lock, true); bool reader_exists = !list_empty(&rcu.cur_preempted); rcu.preempt_blocking_det = reader_exists; irq_spinlock_unlock(&rcu.preempt_lock, true); if (reader_exists) { /* Update statistic. */ ++rcu.stat_preempt_blocking_cnt; return semaphore_down_interruptable(&rcu.remaining_readers); } return true; } /** Called by the scheduler() when switching away from the current thread. */ void rcu_after_thread_ran(void) { ASSERT(interrupts_disabled()); ASSERT(CPU->rcu.pnesting_cnt == &THREAD->rcu.nesting_cnt); /* Preempted a reader critical section for the first time. */ if (0 < THREAD->rcu.nesting_cnt && !THREAD->rcu.was_preempted) { THREAD->rcu.was_preempted = true; irq_spinlock_lock(&rcu.preempt_lock, false); if (CPU->rcu.last_seen_gp != rcu.cur_gp) { /* The reader started before the GP started - we must wait for it.*/ list_append(&THREAD->rcu.preempt_link, &rcu.cur_preempted); } else { /* * The reader started after the GP started and this cpu * already noted a quiescent state. We might block the next GP. */ list_append(&THREAD->rcu.preempt_link, &rcu.next_preempted); } irq_spinlock_unlock(&rcu.preempt_lock, false); } /* * The preempted reader has been noted globally. There are therefore * no readers running on this cpu so this is a quiescent state. */ record_qs(); /* * This cpu is holding up the current GP. Let the detector know * it has just passed a quiescent state. * * The detector waits separately for preempted readers, so we have * to notify the detector even if we have just preempted a reader. */ if (CPU->rcu.is_delaying_gp) { CPU->rcu.is_delaying_gp = false; semaphore_up(&rcu.remaining_readers); } /* * After this point THREAD is 0 and stays 0 until the scheduler() * switches to a new thread. Use a temporary nesting counter for readers * in handlers of interrupts that are raised while idle in the scheduler. */ CPU->rcu.pnesting_cnt = &CPU->rcu.tmp_nesting_cnt; /* * Forcefully associate the detector with the highest priority * even if preempted due to its time slice running out. * * todo: Replace with strict scheduler priority classes. */ if (THREAD == rcu.detector_thr) { THREAD->priority = -1; } else if (THREAD == CPU->rcu.reclaimer_thr) { THREAD->priority = -1; } upd_max_cbs_in_slice(); } static void upd_max_cbs_in_slice(void) { rcu_cpu_data_t *cr = &CPU->rcu; if (cr->arriving_cbs_cnt > cr->last_arriving_cnt) { size_t arrived_cnt = cr->arriving_cbs_cnt - cr->last_arriving_cnt; cr->stat_max_slice_cbs = max(arrived_cnt, cr->stat_max_slice_cbs); } cr->last_arriving_cnt = cr->arriving_cbs_cnt; } /** Called by the scheduler() when switching to a newly scheduled thread. */ void rcu_before_thread_runs(void) { ASSERT(PREEMPTION_DISABLED || interrupts_disabled()); ASSERT(&CPU->rcu.tmp_nesting_cnt == CPU->rcu.pnesting_cnt); CPU->rcu.pnesting_cnt = &THREAD->rcu.nesting_cnt; } /** Prints RCU run-time statistics. */ void rcu_print_stat(void) { /* * Don't take locks. Worst case is we get out-dated values. * CPU local values are updated without any locks, so there * are no locks to lock in order to get up-to-date values. */ printf("Configuration: expedite_threshold=%d, critical_threshold=%d," " detect_sleep=%dms\n", EXPEDITE_THRESHOLD, CRITICAL_THRESHOLD, DETECT_SLEEP_MS); printf("Completed GPs: %" PRIu64 "\n", rcu.completed_gp); printf("Expedited GPs: %zu\n", rcu.stat_expedited_cnt); printf("Delayed GPs: %zu (cpus w/ still running readers after gp sleep)\n", rcu.stat_delayed_cnt); printf("Preempt blocked GPs: %zu (waited for preempted readers; " "running or not)\n", rcu.stat_preempt_blocking_cnt); printf("Smp calls: %zu\n", rcu.stat_smp_call_cnt); printf("Max arrived callbacks per GP and CPU:\n"); for (unsigned int i = 0; i < config.cpu_count; ++i) { printf(" %zu", cpus[i].rcu.stat_max_cbs); } printf("\nAvg arrived callbacks per GP and CPU (nonempty batches only):\n"); for (unsigned int i = 0; i < config.cpu_count; ++i) { printf(" %zu", cpus[i].rcu.stat_avg_cbs); } printf("\nMax arrived callbacks per time slice and CPU:\n"); for (unsigned int i = 0; i < config.cpu_count; ++i) { printf(" %zu", cpus[i].rcu.stat_max_slice_cbs); } printf("\nMissed GP notifications per CPU:\n"); for (unsigned int i = 0; i < config.cpu_count; ++i) { printf(" %zu", cpus[i].rcu.stat_missed_gps); } printf("\nMissed GP notifications per CPU while waking up:\n"); for (unsigned int i = 0; i < config.cpu_count; ++i) { printf(" %zu", cpus[i].rcu.stat_missed_gp_in_wait); } printf("\n"); } /** @} */