Very slow qemu device access

[Matthew Wilcox]

Everything starts going very slowly after this commit:

commit 37f4a24c2469a10a4c16c641671bd766e276cf9f (refs/bisect/bad)
Author: Ming Lei <ming.lei@xxxxxxxxxx>
Date:   Tue Jun 30 22:03:57 2020 +0800

    blk-mq: centralise related handling into blk_mq_get_driver_tag
    
    Move .nr_active update and request assignment into blk_mq_get_driver_tag(),
    all are good to do during getting driver tag.
    
    Meantime blk-flush related code is simplified and flush request needn't
    to update the request table manually any more.
    
    Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
    Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
    Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

By the time xfstests gets to generic/007, things are blocking trying
to get tags:

root@bobo-kvm:~# cat /proc/9530/stack
[<0>] blk_mq_get_tag+0x109/0x250
[<0>] __blk_mq_alloc_request+0x67/0xf0
[<0>] blk_mq_submit_bio+0xee/0x560
[<0>] submit_bio_noacct+0x3a3/0x410
[<0>] submit_bio+0x33/0xf0
[<0>] submit_bh_wbc.isra.0+0x139/0x160
[<0>] block_read_full_page+0x357/0x4a0
[<0>] blkdev_readpage+0x13/0x20
[<0>] do_read_cache_page+0x557/0x860
...

maybe tags aren't getting freed properly?  Or things aren't being woken
up promptly?

(that trace is from current linus head; i bisected back to this commit)