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>
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 6746415..4ae987c 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -35,7 +35,7 @@
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 @@
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,