Messages in this thread Patch in this message | | | From | Lai Jiangshan <> | Subject | [PATCH 3/4 V2] implement per-domain single-thread state machine call_srcu() | Date | Mon, 19 Mar 2012 16:12:13 +0800 |
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o state machine is light way and single-threaded, it is preemptible when checking.
o state machine is a work_struct. So, there is no thread occupied by SRCU when the srcu is not actived(no callback). And it does not sleep(avoid to occupy a thread when sleep).
o state machine is the only thread can flip/check/write(*) the srcu_struct, so we don't need any mutex. (write(*): except ->per_cpu_ref, ->running, ->batch_queue)
o synchronize_srcu() will take the processing ownership when no other state-machine is running.(the task of synchronize_srcu() becomes the state-machine-thread). This fast patch can avoid scheduling. When the fast path fails, it falls back to "call_srcu() + wait".
o callback is probably called in the same CPU when it is queued.
The trip of a callback: 1) ->batch_queue when call_srcu()
2) ->batch_check0 when try to do check_zero
3) ->batch_check1 after finish its first check_zero and the flip
4) ->batch_done after finish its second check_zero
The current requirement of the callbacks: The callback will be called inside process context. The callback should be fast without any sleeping path.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> --- include/linux/srcu.h | 25 ++++- kernel/srcu.c | 310 +++++++++++++++++++++++++++++++++++++++----------- 2 files changed, 268 insertions(+), 67 deletions(-)
diff --git a/include/linux/srcu.h b/include/linux/srcu.h index e5ce804..075fb8c 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -29,16 +29,30 @@ #include <linux/mutex.h> #include <linux/rcupdate.h> +#include <linux/workqueue.h> struct srcu_struct_array { unsigned long c[2]; unsigned long seq[2]; }; +struct rcu_batch { + struct rcu_head *head, **tail; +}; + struct srcu_struct { unsigned completed; struct srcu_struct_array __percpu *per_cpu_ref; - struct mutex mutex; + spinlock_t queue_lock; /* protect ->batch_queue, ->running */ + bool running; + /* callbacks just queued */ + struct rcu_batch batch_queue; + /* callbacks try to do the first check_zero */ + struct rcu_batch batch_check0; + /* callbacks done with the first check_zero and the flip */ + struct rcu_batch batch_check1; + struct rcu_batch batch_done; + struct delayed_work work; #ifdef CONFIG_DEBUG_LOCK_ALLOC struct lockdep_map dep_map; #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ @@ -62,12 +76,21 @@ int init_srcu_struct(struct srcu_struct *sp); #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ +/* + * queue callbacks which will be invoked after grace period. + * The callback will be called inside process context. + * The callback should be fast without any sleeping path. + */ +void call_srcu(struct srcu_struct *sp, struct rcu_head *head, + void (*func)(struct rcu_head *head)); + void cleanup_srcu_struct(struct srcu_struct *sp); int __srcu_read_lock(struct srcu_struct *sp) __acquires(sp); void __srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp); void synchronize_srcu(struct srcu_struct *sp); void synchronize_srcu_expedited(struct srcu_struct *sp); long srcu_batches_completed(struct srcu_struct *sp); +void srcu_barrier(struct srcu_struct *sp); #ifdef CONFIG_DEBUG_LOCK_ALLOC diff --git a/kernel/srcu.c b/kernel/srcu.c index 2923541..c193d59 100644 --- a/kernel/srcu.c +++ b/kernel/srcu.c @@ -34,10 +34,60 @@ #include <linux/delay.h> #include <linux/srcu.h> +static inline void rcu_batch_init(struct rcu_batch *b) +{ + b->head = NULL; + b->tail = &b->head; +} + +static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head) +{ + *b->tail = head; + b->tail = &head->next; +} + +static inline bool rcu_batch_empty(struct rcu_batch *b) +{ + return b->tail == &b->head; +} + +static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b) +{ + struct rcu_head *head; + + if (rcu_batch_empty(b)) + return NULL; + + head = b->head; + b->head = head->next; + if (b->tail == &head->next) + rcu_batch_init(b); + + return head; +} + +static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from) +{ + if (!rcu_batch_empty(from)) { + *to->tail = from->head; + to->tail = from->tail; + rcu_batch_init(from); + } +} + +/* single-thread state-machine */ +static void process_srcu(struct work_struct *work); + static int init_srcu_struct_fields(struct srcu_struct *sp) { sp->completed = 0; - mutex_init(&sp->mutex); + spin_lock_init(&sp->queue_lock); + sp->running = false; + rcu_batch_init(&sp->batch_queue); + rcu_batch_init(&sp->batch_check0); + rcu_batch_init(&sp->batch_check1); + rcu_batch_init(&sp->batch_done); + INIT_DELAYED_WORK(&sp->work, process_srcu); sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); return sp->per_cpu_ref ? 0 : -ENOMEM; } @@ -251,29 +301,22 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock); * we repeatedly block for 1-millisecond time periods. This approach * has done well in testing, so there is no need for a config parameter. */ -#define SYNCHRONIZE_SRCU_READER_DELAY 5 +#define SRCU_RETRY_CHECK_DELAY 5 #define SYNCHRONIZE_SRCU_TRYCOUNT 2 #define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12 /* - * Wait until all the readers(which starts before this wait_idx() - * with the specified idx) complete. + * the caller should ensures the ->completed is not changed while checking + * and idx = (->completed & 1) ^ 1 */ -static void wait_idx(struct srcu_struct *sp, int idx, int trycount) +static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount) { - /* - * SRCU read-side critical sections are normally short, so wait - * a small amount of time before possibly blocking. - */ - if (!srcu_readers_active_idx_check(sp, idx)) { - udelay(SYNCHRONIZE_SRCU_READER_DELAY); - while (!srcu_readers_active_idx_check(sp, idx)) { - if (trycount > 0) { - trycount--; - udelay(SYNCHRONIZE_SRCU_READER_DELAY); - } else - schedule_timeout_interruptible(1); - } + for (;;) { + if (srcu_readers_active_idx_check(sp, idx)) + return true; + if (--trycount <= 0) + return false; + udelay(SRCU_RETRY_CHECK_DELAY); } } @@ -282,12 +325,51 @@ static void srcu_flip(struct srcu_struct *sp) sp->completed++; } +void call_srcu(struct srcu_struct *sp, struct rcu_head *head, + void (*func)(struct rcu_head *head)) +{ + unsigned long flags; + + head->next = NULL; + head->func = func; + spin_lock_irqsave(&sp->queue_lock, flags); + rcu_batch_queue(&sp->batch_queue, head); + if (!sp->running) { + sp->running = true; + queue_delayed_work(system_nrt_wq, &sp->work, 0); + } + spin_unlock_irqrestore(&sp->queue_lock, flags); +} +EXPORT_SYMBOL_GPL(call_srcu); + +struct rcu_synchronize { + struct rcu_head head; + struct completion completion; +}; + +/* + * Awaken the corresponding synchronize_srcu() instance now that a + * grace period has elapsed. + */ +static void wakeme_after_rcu(struct rcu_head *head) +{ + struct rcu_synchronize *rcu; + + rcu = container_of(head, struct rcu_synchronize, head); + complete(&rcu->completion); +} + +static void srcu_advance_batches(struct srcu_struct *sp, int trycount); +static void srcu_reschedule(struct srcu_struct *sp); + /* * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). */ static void __synchronize_srcu(struct srcu_struct *sp, int trycount) { - int busy_idx; + struct rcu_synchronize rcu; + struct rcu_head *head = &rcu.head; + bool done = false; rcu_lockdep_assert(!lock_is_held(&sp->dep_map) && !lock_is_held(&rcu_bh_lock_map) && @@ -295,54 +377,32 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount) !lock_is_held(&rcu_sched_lock_map), "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section"); - mutex_lock(&sp->mutex); - busy_idx = sp->completed & 0X1UL; - - /* - * There are some readers start with idx=0, and some others start - * with idx=1. So two wait_idx()s are enough for synchronize: - * __synchronize_srcu() { - * wait_idx(sp, 0, trycount); - * wait_idx(sp, 1, trycount); - * } - * When it returns, all started readers have complete. - * - * But synchronizer may be starved by the readers, example, - * if sp->complete & 0x1L == 1, wait_idx(sp, 1, expedited) - * may not returns if there are continuous readers start - * with idx=1. - * - * So we need to flip the busy index to keep synchronizer - * from starvation. - */ - - /* - * The above comments assume we have readers with all the - * 2 idx. It does have this probability, some readers may - * hold the reader lock with idx=1-busy_idx: - * - * Suppose that during the previous grace period, a reader - * picked up the old value of the index, but did not increment - * its counter until after the previous instance of - * __synchronize_srcu() did the counter summation and recheck. - * That previous grace period was OK because the reader did - * not start until after the grace period started, so the grace - * period was not obligated to wait for that reader. - * - * Because this probability is not high, wait_idx() - * will normally not need to wait. - */ - wait_idx(sp, 1 - busy_idx, trycount); - - /* flip the index to ensure the return of the next wait_idx() */ - srcu_flip(sp); - - /* - * Now that wait_idx() has waited for the really old readers. - */ - wait_idx(sp, busy_idx, trycount); + init_completion(&rcu.completion); + + head->next = NULL; + head->func = wakeme_after_rcu; + spin_lock_irq(&sp->queue_lock); + if (!sp->running) { + /* steal the processing owner */ + sp->running = true; + rcu_batch_queue(&sp->batch_check0, head); + spin_unlock_irq(&sp->queue_lock); + + srcu_advance_batches(sp, trycount); + if (!rcu_batch_empty(&sp->batch_done)) { + BUG_ON(sp->batch_done.head != head); + rcu_batch_dequeue(&sp->batch_done); + done = true; + } + /* give the processing owner to work_struct */ + srcu_reschedule(sp); + } else { + rcu_batch_queue(&sp->batch_queue, head); + spin_unlock_irq(&sp->queue_lock); + } - mutex_unlock(&sp->mutex); + if (!done) + wait_for_completion(&rcu.completion); } /** @@ -386,6 +446,12 @@ void synchronize_srcu_expedited(struct srcu_struct *sp) } EXPORT_SYMBOL_GPL(synchronize_srcu_expedited); +void srcu_barrier(struct srcu_struct *sp) +{ + synchronize_srcu(sp); +} +EXPORT_SYMBOL_GPL(srcu_barrier); + /** * srcu_batches_completed - return batches completed. * @sp: srcu_struct on which to report batch completion. @@ -399,3 +465,115 @@ long srcu_batches_completed(struct srcu_struct *sp) return sp->completed; } EXPORT_SYMBOL_GPL(srcu_batches_completed); + +#define SRCU_CALLBACK_BATCH 10 +#define SRCU_INTERVAL 1 + +static void srcu_collect_new(struct srcu_struct *sp) +{ + if (!rcu_batch_empty(&sp->batch_queue)) { + spin_lock_irq(&sp->queue_lock); + rcu_batch_move(&sp->batch_check0, &sp->batch_queue); + spin_unlock_irq(&sp->queue_lock); + } +} + +static void srcu_advance_batches(struct srcu_struct *sp, int trycount) +{ + int idx = 1 ^ (sp->completed & 1); + + /* + * There are some readers start with idx=0, and some others start + * with idx=1. So two success try_check_zero()s (with idx=0,and idx=1) + * are enough for a callback to complete. + */ + + if (rcu_batch_empty(&sp->batch_check0) && + rcu_batch_empty(&sp->batch_check1)) + return; /* no callbacks need to be advanced */ + + if (!try_check_zero(sp, idx, trycount)) + return; /* failed to advance, will try after SRCU_INTERVAL */ + + /* + * The callbacks in ->batch_check1 have already done with their + * first check zero and a flip after check. Their first + * check zero is "try_check_zero(sp, idx^1, ...)", + * and they finish try_check_zero(sp, idx, ...) just now. + * So they all completed, move them to ->batch_done. + */ + rcu_batch_move(&sp->batch_done, &sp->batch_check1); + + if (rcu_batch_empty(&sp->batch_check0)) + return; /* no callbacks need to be advanced */ + srcu_flip(sp); + + /* + * The callbacks in ->batch_check0 just finish their + * first check zero and a flip after check, move them to + * ->batch_check1 for future checking with a different idx. + */ + rcu_batch_move(&sp->batch_check1, &sp->batch_check0); + + /* + * SRCU read-side critical sections are normally short, so check + * twice after a flip. + */ + trycount = trycount < 2 ? 2 : trycount; + if (!try_check_zero(sp, idx^1, trycount)) + return; /* failed to advance, will try after SRCU_INTERVAL */ + + /* + * The callbacks in ->batch_check1 have already done with their + * first check zero and a flip after check. Their first + * check zero is "try_check_zero(sp, idx, ...)", + * and they finish try_check_zero(sp, idx^1, ...) just now. + * So they all completed, move them to ->batch_done. + */ + rcu_batch_move(&sp->batch_done, &sp->batch_check1); +} + +static void srcu_invoke_callbacks(struct srcu_struct *sp) +{ + int i; + struct rcu_head *head; + + for (i = 0; i < SRCU_CALLBACK_BATCH; i++) { + head = rcu_batch_dequeue(&sp->batch_done); + if (!head) + break; + head->func(head); + } +} + +static void srcu_reschedule(struct srcu_struct *sp) +{ + bool pending = true; + + if (rcu_batch_empty(&sp->batch_done) && + rcu_batch_empty(&sp->batch_check1) && + rcu_batch_empty(&sp->batch_check0) && + rcu_batch_empty(&sp->batch_queue)) { + spin_lock_irq(&sp->queue_lock); + if (rcu_batch_empty(&sp->batch_queue)) { + sp->running = false; + pending = false; + } + spin_unlock_irq(&sp->queue_lock); + } + + if (pending) + queue_delayed_work(system_nrt_wq, &sp->work, SRCU_INTERVAL); +} + +static void process_srcu(struct work_struct *work) +{ + struct srcu_struct *sp; + + sp = container_of(work, struct srcu_struct, work.work); + + srcu_collect_new(sp); + srcu_advance_batches(sp, 1); + srcu_invoke_callbacks(sp); + srcu_reschedule(sp); +} -- 1.7.4.4
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