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authorJens Axboe <axboe@kernel.dk>2017-11-09 08:32:43 -0700
committerJens Axboe <axboe@kernel.dk>2017-11-10 19:53:25 -0700
commiteb619fdb2d4cb8b3d3419e9113921e87e7daf557 (patch)
tree491c0230e3ce0cead62bf59b090bf6c8b2bbd6e0 /block
parente454d122e22826589cfa08788a802ab09d4fae24 (diff)
downloadlinux-stericsson-eb619fdb2d4cb8b3d3419e9113921e87e7daf557.tar.gz
blk-mq: fix issue with shared tag queue re-running
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. Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com> Tested-by: Bart Van Assche <bart.vanassche@wdc.com> Reviewed-by: Ming Lei <ming.lei@redhat.com> Reviewed-by: Omar Sandoval <osandov@fb.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block')
-rw-r--r--block/blk-mq-debugfs.c1
-rw-r--r--block/blk-mq.c85
2 files changed, 48 insertions, 38 deletions
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..fed8165973a3 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -998,49 +998,64 @@ done:
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 ourselves 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
@@ -1155,8 +1163,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
* returning BLK_STS_RESOURCE. Two exceptions are scsi-mq
* and dm-rq.
*/
- if (!blk_mq_sched_needs_restart(hctx) &&
- !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
+ if (!blk_mq_sched_needs_restart(hctx) ||
+ (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;