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blk-mq: use sbq wait queues instead of restart for driver tags

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Commit 50e1dab86a ("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@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Omar Sandoval
2017-02-22 10:58:29 -08:00
committed by Jens Axboe
parent 2d19020b08
commit da55f2cc78
2 changed files with 57 additions and 9 deletions
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2
include/linux/blk-mq.h
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@@ -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,