Hi,
在 2024/08/19 11:49, Muchun Song 写道:
On Mon, Aug 19, 2024 at 10:28 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
Hi Muchun,
On Sun, Aug 11, 2024 at 06:19:19PM +0800, Muchun Song wrote:
Supposing the following scenario with a virtio_blk driver.
CPU0 CPU1
blk_mq_try_issue_directly()
__blk_mq_issue_directly()
q->mq_ops->queue_rq()
virtio_queue_rq()
blk_mq_stop_hw_queue()
virtblk_done()
blk_mq_request_bypass_insert() blk_mq_start_stopped_hw_queues()
/* Add IO request to dispatch list */ 1) store blk_mq_start_stopped_hw_queue()
clear_bit(BLK_MQ_S_STOPPED) 3) store
blk_mq_run_hw_queue() blk_mq_run_hw_queue()
if (!blk_mq_hctx_has_pending()) if (!blk_mq_hctx_has_pending()) 4) load
return return
blk_mq_sched_dispatch_requests() blk_mq_sched_dispatch_requests()
if (blk_mq_hctx_stopped()) 2) load if (blk_mq_hctx_stopped())
return return
__blk_mq_sched_dispatch_requests() __blk_mq_sched_dispatch_requests()
The full memory barrier should be inserted between 1) and 2), as well as between
3) and 4) to make sure that either CPU0 sees BLK_MQ_S_STOPPED is cleared or CPU1
sees dispatch list or setting of bitmap of software queue. Otherwise, either CPU
will not re-run the hardware queue causing starvation.
Yeah, it is one kind of race which is triggered when adding request into
->dispatch list after returning STS_RESOURCE. We were troubled by lots of
such kind of race.
Yes. I saw the similar fix for BLK_MQ_S_SCHED_RESTART.
stopping queue is used in very less drivers, and its only purpose should
be for throttling hw queue in case that low level queue is busy. There seems
more uses of blk_mq_stop_hw_queues(), but most of them should be replaced
with blk_mq_quiesce_queue().
IMO, fixing this kind of issue via memory barrier is too tricky to
maintain cause WRITE/READ dependency is very hard to follow. I'd suggest to
make memory barrier solution as the last resort, and we can try to figure
out other easier & more reliable way first.
I do agree it is hard to maintain the dependencies in the future. We should
propose an easy-maintainable solution. But I thought it is a long-term issue
throughout different stable linux distros. Adding a mb is the easy way to fix
the problem (the code footprint is really small), so it will be very
easy for others
to backport those bug fixes to different stable linux distros. Therefore, mb
should be an interim solution. Then, we could improve it based on the solution
you've proposed below. What do you think?
I'll agree with Ming, let's figure out a better fix first. Easy to
backport to stables is not first consideration.
Thanks,
Muchun.
One idea I thought of is to call blk_mq_request_bypass_insert()(or rename
& export it) before calling blk_mq_stop_hw_queue() in driver, then
return new status code STS_STOP_DISPATCH for notifying blk-mq to stop
dispatch simply.
New status code look good to me, however, I wonder can we just remove
the problematic blk_mq_stop_hw_queue(), and replace it by handling the
new status from block layer?
- Passing the new status to blk_mq_run_dispatch_ops, and quiesce with
the new status, if no request is inflight, unquiesce immediately;
- unquiesce is any IO is done afterwards;
Thanks,
Kuai
thanks,
Ming
.
