[PATCHv4 0/3] block: move sched_tags allocation/de-allocation outside of locking context

[Nilay Shroff <nilay@linux.ibm.com>]

Hi,

There have been a few reports[1] indicating potential lockdep warnings due
to a lock dependency from the percpu allocator to the elevator lock. This
patch series aims to eliminate that dependency.

The series consists of three patches:
The first patch is preparatory patch and just move elevator queue
allocation logic from ->init_sched into blk_mq_init_sched.

The second patch in the series restructures sched_tags allocation and
deallocation during elevator update/switch operations to ensure these
actions are performed entirely outside the ->freeze_lock and ->elevator_
lock. This eliminates the percpu allocator’s lock dependency on the
elevator and freeze lock during scheduler transitions.

The third patch introduces batch allocation and deallocation helpers for
sched_tags. These helpers are used during __blk_mq_update_nr_hw_queues()
to decouple sched_tags memory management from both the elevator and freeze
locks, addressing the lockdep concerns in the nr_hw_queues update path.

[1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/

Changes since v3:
    - Further split the patchset into three patch series so that we can
      have a separate patch for sched_tags batch allocation/deallocation
      (Ming Lei)
    - Use Xarray to store and load the sched_tags (Ming Lei)
    - Unexport elevator_alloc() as we no longer need to use it outside
      of block layer core (hch)
    - unwind the sched_tags allocation and free tags when we it fails in
      the middle of allocation (hch)
    - Move struct elevator_queue header from commin header to elevator.c
      as there's no user of it outside elevator.c (Ming Lei, hch)
Link to v3: https://lore.kernel.org/all/20250616173233.3803824-1-nilay@linux.ibm.com/

Change since v2:
    - Split the patch into a two patch series. The first patch updates
      ->init_sched elevator API change and second patch handles the sched
      tags allocation/de-allocation logic (Ming Lei)
    - Address sched tags allocation/deallocation logic while running in the
      context of nr_hw_queue update so that we can handle all possible
      scenarios in a single patchest (Ming Lei)
Link to v2: https://lore.kernel.org/all/20250528123638.1029700-1-nilay@linux.ibm.com/

Changes since v1:
    - As the lifetime of elevator queue and sched tags are same, allocate
      and move sched tags under struct elevator_queue (Ming Lei)
Link to v1: https://lore.kernel.org/all/20250520103425.1259712-1-nilay@linux.ibm.com/

Nilay Shroff (3):
  block: move elevator queue allocation logic into blk_mq_init_sched
  block: fix lockdep warning caused by lock dependency in
    elv_iosched_store
  block: fix potential deadlock while running nr_hw_queue update

 block/bfq-iosched.c   |  13 +--
 block/blk-mq-sched.c  | 238 +++++++++++++++++++++++++++++-------------
 block/blk-mq-sched.h  |  19 +++-
 block/blk-mq.c        |  12 ++-
 block/blk.h           |   3 +-
 block/elevator.c      |  52 +++++++--
 block/elevator.h      |  21 +++-
 block/kyber-iosched.c |  11 +-
 block/mq-deadline.c   |  14 +--
 9 files changed, 267 insertions(+), 116 deletions(-)

-- 
2.49.0