From: Omar Sandoval <[email protected]> Commit 50e1dab86aa2 ("blk-mq-sched: fix starvation for multiple hardware queues and shared tags") fixed one starvation issue for shared tags. However, we can still get into a situation where we fail to allocate a tag because all tags are allocated but we don't have any pending requests on any hardware queue.
One solution for this would be to restart all queues that share a tag map, but that really sucks. Ideally, we could just block and wait for a tag, but that isn't always possible from blk_mq_dispatch_rq_list(). However, we can still use the struct sbitmap_queue wait queues with a custom callback instead of blocking. This has a few benefits: 1. It avoids iterating over all hardware queues when completing an I/O, which the current restart code has to do. 2. It benefits from the existing rolling wakeup code. 3. It avoids punting to another thread just to have it block. Signed-off-by: Omar Sandoval <[email protected]> --- block/blk-mq.c | 60 ++++++++++++++++++++++++++++++++++++++++++++------ include/linux/blk-mq.h | 2 ++ 2 files changed, 55 insertions(+), 7 deletions(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index 5564a9d103ca..0dacb743d4d7 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -904,6 +904,44 @@ static bool reorder_tags_to_front(struct list_head *list) return first != NULL; } +static int blk_mq_dispatch_wake(wait_queue_t *wait, unsigned mode, int flags, + void *key) +{ + struct blk_mq_hw_ctx *hctx; + + hctx = container_of(wait, struct blk_mq_hw_ctx, dispatch_wait); + + list_del(&wait->task_list); + clear_bit_unlock(BLK_MQ_S_TAG_WAITING, &hctx->state); + blk_mq_run_hw_queue(hctx, true); + return 1; +} + +static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx) +{ + struct sbq_wait_state *ws; + + /* + * The TAG_WAITING bit serves as a lock protecting hctx->dispatch_wait. + * The thread which wins the race to grab this bit adds the hardware + * queue to the wait queue. + */ + if (test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) || + test_and_set_bit_lock(BLK_MQ_S_TAG_WAITING, &hctx->state)) + return false; + + init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake); + ws = bt_wait_ptr(&hctx->tags->bitmap_tags, hctx); + + /* + * As soon as this returns, it's no longer safe to fiddle with + * hctx->dispatch_wait, since a completion can wake up the wait queue + * and unlock the bit. + */ + add_wait_queue(&ws->wait, &hctx->dispatch_wait); + return true; +} + bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list) { struct request_queue *q = hctx->queue; @@ -926,20 +964,27 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list) struct blk_mq_queue_data bd; rq = list_first_entry(list, struct request, queuelist); - if (!blk_mq_get_driver_tag(rq, &hctx, false)) { + if (!blk_mq_get_driver_tag(rq, NULL, false)) { if (!queued && reorder_tags_to_front(list)) continue; /* - * We failed getting a driver tag. Mark the queue(s) - * as needing a restart. Retry getting a tag again, - * in case the needed IO completed right before we - * marked the queue as needing a restart. + * The initial allocation attempt failed, so we need to + * rerun the hardware queue when a tag is freed. */ - blk_mq_sched_mark_restart(hctx); - if (!blk_mq_get_driver_tag(rq, &hctx, false)) + if (blk_mq_dispatch_wait_add(hctx)) { + /* + * It's possible that a tag was freed in the + * window between the allocation failure and + * adding the hardware queue to the wait queue. + */ + if (!blk_mq_get_driver_tag(rq, NULL, false)) + break; + } else { break; + } } + list_del_init(&rq->queuelist); bd.rq = rq; @@ -1051,6 +1096,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx) void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) { if (unlikely(blk_mq_hctx_stopped(hctx) || + test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) || !blk_mq_hw_queue_mapped(hctx))) return; diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h index 8e4df3d6c8cd..001d30d727c5 100644 --- a/include/linux/blk-mq.h +++ b/include/linux/blk-mq.h @@ -33,6 +33,7 @@ struct blk_mq_hw_ctx { struct blk_mq_ctx **ctxs; unsigned int nr_ctx; + wait_queue_t dispatch_wait; atomic_t wait_index; struct blk_mq_tags *tags; @@ -160,6 +161,7 @@ enum { BLK_MQ_S_STOPPED = 0, BLK_MQ_S_TAG_ACTIVE = 1, BLK_MQ_S_SCHED_RESTART = 2, + BLK_MQ_S_TAG_WAITING = 3, BLK_MQ_MAX_DEPTH = 10240, -- 2.11.1
