This patch attempts to make the case of hctx re-running on driver tag
failure more robust. Without this patch, it's pretty easy to trigger a
stall condition with shared tags. An example is using null_blk like
this:
modprobe null_blk queue_mode=2 nr_devices=4 shared_tags=1 submit_queues=1 hw_queue_depth=1
which sets up 4 devices, sharing the same tag set with a depth of 1.
Running a fio job ala:
[global]
bs=4k
rw=randread
norandommap
direct=1
ioengine=libaio
iodepth=4
[nullb0]
filename=/dev/nullb0
[nullb1]
filename=/dev/nullb1
[nullb2]
filename=/dev/nullb2
[nullb3]
filename=/dev/nullb3
will inevitably end with one or more threads being stuck waiting for a
scheduler tag. That IO is then stuck forever, until someone else
triggers a run of the queue.
Ensure that we always re-run the hardware queue, if the driver tag we
were waiting for got freed before we added our leftover request entries
back on the dispatch list.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 7f4a1ba532af..bb7f08415203 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -179,7 +179,6 @@ static const char *const hctx_state_name[] = {
HCTX_STATE_NAME(STOPPED),
HCTX_STATE_NAME(TAG_ACTIVE),
HCTX_STATE_NAME(SCHED_RESTART),
- HCTX_STATE_NAME(TAG_WAITING),
HCTX_STATE_NAME(START_ON_RUN),
};
#undef HCTX_STATE_NAME
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3d759bb8a5bb..8dc5db40df9d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -998,49 +998,64 @@ bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
return rq->tag != -1;
}
-static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode, int flags,
- void *key)
+static int blk_mq_dispatch_wake(wait_queue_entry_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->entry);
- clear_bit_unlock(BLK_MQ_S_TAG_WAITING, &hctx->state);
+ list_del_init(&wait->entry);
blk_mq_run_hw_queue(hctx, true);
return 1;
}
-static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)
+static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx,
+ struct request *rq)
{
+ struct blk_mq_hw_ctx *this_hctx = *hctx;
+ wait_queue_entry_t *wait = &this_hctx->dispatch_wait;
struct sbq_wait_state *ws;
+ if (!list_empty_careful(&wait->entry))
+ return false;
+
+ spin_lock(&this_hctx->lock);
+ if (!list_empty(&wait->entry)) {
+ spin_unlock(&this_hctx->lock);
+ return false;
+ }
+
+ ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
+ add_wait_queue(&ws->wait, wait);
+
/*
- * 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.
+ * 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 (test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) ||
- test_and_set_bit_lock(BLK_MQ_S_TAG_WAITING, &hctx->state))
+ if (!blk_mq_get_driver_tag(rq, hctx, false)) {
+ spin_unlock(&this_hctx->lock);
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.
+ * We got a tag, remove outselves from the wait queue to ensure
+ * someone else gets the wakeup.
*/
- add_wait_queue(&ws->wait, &hctx->dispatch_wait);
+ spin_lock_irq(&ws->wait.lock);
+ list_del_init(&wait->entry);
+ spin_unlock_irq(&ws->wait.lock);
+ spin_unlock(&this_hctx->lock);
return true;
}
bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
- bool got_budget)
+ bool got_budget)
{
struct blk_mq_hw_ctx *hctx;
struct request *rq, *nxt;
+ bool no_tag = false;
int errors, queued;
if (list_empty(list))
@@ -1060,22 +1075,15 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
if (!blk_mq_get_driver_tag(rq, &hctx, false)) {
/*
* The initial allocation attempt failed, so we need to
- * rerun the hardware queue when a tag is freed.
+ * rerun the hardware queue when a tag is freed. The
+ * waitqueue takes care of that. If the queue is run
+ * before we add this entry back on the dispatch list,
+ * we'll re-run it below.
*/
- if (!blk_mq_dispatch_wait_add(hctx)) {
- if (got_budget)
- blk_mq_put_dispatch_budget(hctx);
- break;
- }
-
- /*
- * 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, &hctx, false)) {
+ if (!blk_mq_dispatch_wait_add(&hctx, rq)) {
if (got_budget)
blk_mq_put_dispatch_budget(hctx);
+ no_tag = true;
break;
}
}
@@ -1140,10 +1148,10 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
* it is no longer set that means that it was cleared by another
* thread and hence that a queue rerun is needed.
*
- * If TAG_WAITING is set that means that an I/O scheduler has
- * been configured and another thread is waiting for a driver
- * tag. To guarantee fairness, do not rerun this hardware queue
- * but let the other thread grab the driver tag.
+ * If 'no_tag' is set, that means that we failed getting
+ * a driver tag with an I/O scheduler attached. If our dispatch
+ * waitqueue is no longer active, ensure that we run the queue
+ * AFTER adding our entries back to the list.
*
* If no I/O scheduler has been configured it is possible that
* the hardware queue got stopped and restarted before requests
@@ -1156,7 +1164,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
* and dm-rq.
*/
if (!blk_mq_sched_needs_restart(hctx) &&
- !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
+ (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
blk_mq_run_hw_queue(hctx, true);
}
@@ -2020,6 +2028,9 @@ static int blk_mq_init_hctx(struct request_queue *q,
hctx->nr_ctx = 0;
+ init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake);
+ INIT_LIST_HEAD(&hctx->dispatch_wait.entry);
+
if (set->ops->init_hctx &&
set->ops->init_hctx(hctx, set->driver_data, hctx_idx))
goto free_bitmap;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 674641527da7..4ae987c2352c 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -35,7 +35,7 @@ struct blk_mq_hw_ctx {
struct blk_mq_ctx **ctxs;
unsigned int nr_ctx;
- wait_queue_entry_t dispatch_wait;
+ wait_queue_entry_t dispatch_wait;
atomic_t wait_index;
struct blk_mq_tags *tags;
@@ -181,8 +181,7 @@ 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_S_START_ON_RUN = 4,
+ BLK_MQ_S_START_ON_RUN = 3,
BLK_MQ_MAX_DEPTH = 10240,
--
Jens Axboe
