blk-mq: share one tags_srcu instance across all tag sets

Freed request pages and flush queues are handed to call_srcu() so that
concurrent tag iterators never see them disappear. Each tag set embeds
its own srcu_struct for this, which blk_mq_free_tag_set() has to drain
with srcu_barrier() before cleanup_srcu_struct(). The barrier waits for
a full SRCU grace period even when the callbacks are long done, adding
1-3 ms to every tag set teardown. Drivers that free tag sets under a
lock make it worse: ublk does it under the global ublk_ctl_mutex, so
deleting devices from many threads serializes on that grace period.

The callbacks free memory that only blk-mq owns. They touch neither
the tag set nor driver code, so they don't need a per-set domain.
Replace the per-set srcu_struct with a single global one. Freeing a tag
set no longer waits for a grace period, and each tag set no longer
allocates per-CPU SRCU state. The read side is unchanged. The only cost
is that a long-running iterator can now delay these frees for other tag
sets too, which only postpones freeing memory.

On 7.3-rc5 in a 16-vCPU VM, with 1 queue and depth 16 per ublk device,
median of 3 runs (5 for the 16-thread null_blk case):

                                       before     after
  ublk DEL_DEV p50, serial             3.45 ms    0.17 ms
  ublk delete 1000 devices, serial     19.5 s     14.9 s
  ublk delete 4000 devices, 16 thr     19.0 s     2.9 s
  null_blk destroy 500 devs, serial    9.97 s     9.25 s
  null_blk destroy 256 devs, 16 thr    5.15 s     4.30 s

Fixes: ad0d05dbddc1 ("blk-mq: Defer freeing of tags page_list to SRCU callback")
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://patch.msgid.link/20261001-bug-blk-mq-global-tags-srcu-v1-1-e8415da10d1b@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions