From: Omar Sandoval <osandov@xxxxxx>
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 <osandov@xxxxxx>
---
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
