On Sun, Jun 24, 2018 at 6:16 PM, Ming Lei [off-list ref] wrote:

Hi Guys,

Recently, I am figuring out solutions for removing synchronize_rcu() from
blk_cleanup_queue() so that no long delay is caused during SCSI lun probe[1],
especially from blk_mq_del_queue_tag_set(). This synchronize_rcu() is added
by commit 705cda97ee3abb6ea(blk-mq: Make it safe to use RCU to iterate over
blk_mq_tag_set.tag_list), and commit 6d8c6c0f97ad ("blk-mq: Restart a single
queue if tag sets are shared"), and call this as 'TAG_SHARED in restart'.

Basically speaking, the synchronize_rcu() can't be removed if we have
to restart all tags-shared queues in current way('TAG_SHARED in restart')
when one request is completed. Meantime blk-mq has used blk_mq_mark_tag_wait()
to deal with cross-queue driver tag allocation, which means the two mechanism
are  highly overlapped. Also SCSI has built-in RESTART, and not need
'TAG_SHARED in restart'.

We tried to remove shared-tag restart in 358a3a6bccb7 (blk-mq: don't handle
TAG_SHARED in restart) before, but it is reverted in commit 05b79413946d
(Revert "blk-mq: don't handle TAG_SHARED in restart") because it causes
regression in Bart's SRP test.

Now I am revisiting 'TAG_SHARED in restart' again for the long delay issue
of SCSI LUN probe. And found there is one bug in blk_mq_mark_tag_wait():

- hctx->dispatch_wait is added to wait queue by holding hctx->lock and
the wait queue's lock

- no hctx->lock is held when removing hctx->dispatch_wait from wait
  queue.

- so the two actions(add, remove) may happen meantime since
  hctx->dispatch_wait can be added to different wait queues.

Turns out this issue can be observed easily by applying the patches[2],
which is for removing 'TAG_SHARED in restart', then run simple shared-tag
null_blk test[4].

But if the hctx->lock is held in blk_mq_dispatch_wake(), as done in
patch [3], there isn't such issue at all, so it shows this issue is
related with hctx->lock, and adding/removing hctx->dispatch_wait to
wait queue. But the way of holding hctx->lock in irq context may not
be one accepted solution, since it has been avoided from the beginning
of blk-mq.

So does anyone have better ideas for this issue?

So far, follows what I thought of:

1) fix the mechanism of blk_mq_mark_tag_wait(), and removing
'TAG_SHARED in restart', then we can fix the long delay issue of
SCSI LUN probe, meantime performance can got improved, as I observed,
this way may improve IOPS by 20~30% in multi-LUN scsi_debug test.
But the issue is how to fix?

2) keep 'TAG_SHARED in restart' and let it cover the issue of
blk_mq_mark_tag_wait() as now, then try to improve 'TAG_SHARED in restart'
in another way, so that performance can be better, and synchronize_rcu()
can be removed from blk_mq_del_queue_tag_set(), then SCSI LUN probe long
delay can be fixed. I had wrote patches to do that last year. If anyone
is interested, I may post it out.

Or other ideas, any comments & ideas are welcome!

Looks introducing one new lock to protect hctx->dispatch_wait is fine.


Thanks,
Ming Lei