[PATCH v6 0/8] nbd: eliminate queue freeze/unfreeze overhead in connection setup

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

v5->v6:
1. patch 3, reset stale FEAT*, not only write cache
2. patch 5, using the way Kuai advise to skip freeze
3. patch 6, adopt kuai's suggestion
4. patch 8, rename nr_hw_queues and add some comments

v4->v5:
1. add patch 3 to reset write cache on disconnect
2. fix ("nbd: factor out a nbd_genl_foreach_sock")

v3->v4:
1. factor out nbd_genl_foreach_sock to cleanup duplicated code
2. rewrite some commit message to make what's going more clearly

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


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

Yang Erkun (7):
  nbd: disallow NBD_SET_SOCK on an active device
  nbd: clear queue limits on disconnect
  nbd: remove queue freeze in nbd_add_socket
  nbd: skip queue freeze when setting size at device startup
  nbd: factor out a nbd_genl_foreach_sock
  nbd: remove queue freeze for newly created nbd from netlink path
  nbd: add pre_defined_connections module parameter for pre-created
    devices

 drivers/block/nbd.c | 255 +++++++++++++++++++++++---------------------
 1 file changed, 136 insertions(+), 119 deletions(-)

-- 
2.52.0