[PATCHv7 0/3] block: move sched_tags allocation/de-allocation outside of locking context

Nilay Shroff
2025-07-01T08:20:09Z

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 v6:
- Add warning when loading elevator tags from an xarray yields nothing
(Hannes Reinecke)
- Use elevator tags instead of xarray table as a function argument to
elv_update_nr_hw_queues (Ming Lei)
Link to v6: https://lore.kernel.org/all/20250630054756.54532-1-nilay@linux.ibm.com/

Changes since v5:
- Fixed smatch warning reported by kernel test robot here:
https://lore.kernel.org/all/202506300509.2S1tygch-lkp@intel.com/
Link to v5: https://lore.kernel.org/all/20250627175544.1063910-1-nilay@linux.ibm.com/

Changes since v4:
- Define a local Xarray variable in __blk_mq_update_nr_hw_queues to store
sched_tags, instead of storing it in an Xarray defined in 'struct elevator_tags'
(Ming Lei)
Link to v4: https://lore.kernel.org/all/20250624131716.630465-1-nilay@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 | 223 ++++++++++++++++++++++++++++--------------
block/blk-mq-sched.h | 12 ++-
block/blk-mq.c | 16 ++-
block/blk.h | 2 +-
block/elevator.c | 50 ++++++++--
block/elevator.h | 16 ++-
block/kyber-iosched.c | 11 +--
block/mq-deadline.c | 14 +--
9 files changed, 241 insertions(+), 116 deletions(-)

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2.50.0