From fd5db42254518fbf241dc454e918598fbe494fa2 Mon Sep 17 00:00:00 2001 From: Manfred Spraul Date: Wed, 26 May 2010 14:43:40 -0700 Subject: [PATCH] ipc/sem.c: optimize update_queue() for bulk wakeup calls The following series of patches tries to fix the spinlock contention reported by Chris Mason - his benchmark exposes problems of the current code: - In the worst case, the algorithm used by update_queue() is O(N^2). Bulk wake-up calls can enter this worst case. The patch series fix that. Note that the benchmark app doesn't expose the problem, it just should be fixed: Real world apps might do the wake-ups in another order than perfect FIFO. - The part of the code that runs within the semaphore array spinlock is significantly larger than necessary. The patch series fixes that. This change is responsible for the main improvement. - The cacheline with the spinlock is also used for a variable that is read in the hot path (sem_base) and for a variable that is unnecessarily written to multiple times (sem_otime). The last step of the series cacheline-aligns the spinlock. This patch: The SysV semaphore code allows to perform multiple operations on all semaphores in the array as atomic operations. After a modification, update_queue() checks which of the waiting tasks can complete. The algorithm that is used to identify the tasks is O(N^2) in the worst case. For some cases, it is simple to avoid the O(N^2). The patch adds a detection logic for some cases, especially for the case of an array where all sleeping tasks are single sembuf operations and a multi-sembuf operation is used to wake up multiple tasks. A big database application uses that approach. The patch fixes wakeup due to semctl(,,SETALL,) - the initial version of the patch breaks that. [akpm@linux-foundation.org: make do_smart_update() static] Signed-off-by: Manfred Spraul Cc: Chris Mason Cc: Zach Brown Cc: Jens Axboe Cc: Nick Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- ipc/sem.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 97 insertions(+), 13 deletions(-) diff --git a/ipc/sem.c b/ipc/sem.c index dbef95b15941..81a9c74ab64c 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -434,6 +434,69 @@ static void unlink_queue(struct sem_array *sma, struct sem_queue *q) sma->complex_count--; } +/** check_restart(sma, q) + * @sma: semaphore array + * @q: the operation that just completed + * + * update_queue is O(N^2) when it restarts scanning the whole queue of + * waiting operations. Therefore this function checks if the restart is + * really necessary. It is called after a previously waiting operation + * was completed. + */ +static int check_restart(struct sem_array *sma, struct sem_queue *q) +{ + struct sem *curr; + struct sem_queue *h; + + /* if the operation didn't modify the array, then no restart */ + if (q->alter == 0) + return 0; + + /* pending complex operations are too difficult to analyse */ + if (sma->complex_count) + return 1; + + /* we were a sleeping complex operation. Too difficult */ + if (q->nsops > 1) + return 1; + + curr = sma->sem_base + q->sops[0].sem_num; + + /* No-one waits on this queue */ + if (list_empty(&curr->sem_pending)) + return 0; + + /* the new semaphore value */ + if (curr->semval) { + /* It is impossible that someone waits for the new value: + * - q is a previously sleeping simple operation that + * altered the array. It must be a decrement, because + * simple increments never sleep. + * - The value is not 0, thus wait-for-zero won't proceed. + * - If there are older (higher priority) decrements + * in the queue, then they have observed the original + * semval value and couldn't proceed. The operation + * decremented to value - thus they won't proceed either. + */ + BUG_ON(q->sops[0].sem_op >= 0); + return 0; + } + /* + * semval is 0. Check if there are wait-for-zero semops. + * They must be the first entries in the per-semaphore simple queue + */ + h = list_first_entry(&curr->sem_pending, struct sem_queue, simple_list); + BUG_ON(h->nsops != 1); + BUG_ON(h->sops[0].sem_num != q->sops[0].sem_num); + + /* Yes, there is a wait-for-zero semop. Restart */ + if (h->sops[0].sem_op == 0) + return 1; + + /* Again - no-one is waiting for the new value. */ + return 0; +} + /** * update_queue(sma, semnum): Look for tasks that can be completed. @@ -469,7 +532,7 @@ static void update_queue(struct sem_array *sma, int semnum) again: walk = pending_list->next; while (walk != pending_list) { - int error, alter; + int error, restart; q = (struct sem_queue *)((char *)walk - offset); walk = walk->next; @@ -494,22 +557,43 @@ again: unlink_queue(sma, q); - /* - * The next operation that must be checked depends on the type - * of the completed operation: - * - if the operation modified the array, then restart from the - * head of the queue and check for threads that might be - * waiting for the new semaphore values. - * - if the operation didn't modify the array, then just - * continue. - */ - alter = q->alter; + if (error) + restart = 0; + else + restart = check_restart(sma, q); + wake_up_sem_queue(q, error); - if (alter && !error) + if (restart) goto again; } } +/** do_smart_update(sma, sops, nsops): Optimized update_queue + * @sma: semaphore array + * @sops: operations that were performed + * @nsops: number of operations + * + * do_smart_update() does the required called to update_queue, based on the + * actual changes that were performed on the semaphore array. + */ +static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops) +{ + int i; + + if (sma->complex_count || sops == NULL) { + update_queue(sma, -1); + return; + } + + for (i = 0; i < nsops; i++) { + if (sops[i].sem_op > 0 || + (sops[i].sem_op < 0 && + sma->sem_base[sops[i].sem_num].semval == 0)) + update_queue(sma, sops[i].sem_num); + } +} + + /* The following counts are associated to each semaphore: * semncnt number of tasks waiting on semval being nonzero * semzcnt number of tasks waiting on semval being zero @@ -1225,7 +1309,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current)); if (error <= 0) { if (alter && error == 0) - update_queue(sma, (nsops == 1) ? sops[0].sem_num : -1); + do_smart_update(sma, sops, nsops); goto out_unlock_free; } -- 2.20.1