[PATCH v3 0/6] nbd: eliminate queue freeze/unfreeze overhead in connection setup

[Yang Erkun <yangerkun@huawei.com>]

This series eliminates the blk_mq_freeze_queue()/unfreeze_queue()
overhead that currently occurs during nbd connection setup(nbd provide
ondemand and was using in AI sandbox e2b[1] to mount and attach rootfs).
Each freeze cycle will introduce a RCU grace which was significant
with multi-core system.

time nbd-client --name myexport --connections 96 127.0.0.1 1234

After this patchset:
real    0m0.090s
user    0m0.004s
sys     0m0.018s

Without this patchset:
real    0m2.195s
user    0m0.005s
sys     0m0.022s

v2->v3:
1. Stop using xarray, and protect it by rejecting late NBD_SET_SOCKS
2. Eliminate freeze in nbd_set_size when device setup
3. Add module parameter to eliminate freeze for pre-created device

v1->v2:
1. Rewrite cover letter
2. Eliminate freeze call from blk_mq_update_nr_hw_queues in
   nbd_start_device for netlink path


[1]. https://github.com/e2b-dev/e2b

Long Li (1):
  nbd: simplify find_fallback() by removing redundant logic

Yang Erkun (5):
  nbd: disallow NBD_SET_SOCK on an active device
  nbd: remove queue freeze in nbd_add_socket
  nbd: set nr_hw_queues at device creation to skip queue freeze
  nbd: skip queue freeze when setting size at device startup
  nbd: add nr_hw_queues module parameter for pre-created devices

 drivers/block/nbd.c | 119 ++++++++++++++++++++++++++++----------------
 1 file changed, 76 insertions(+), 43 deletions(-)

-- 
2.52.0