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@xxxxxxxxxxxxx/ 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@xxxxxxxxxxxxx/ Changes since v5: - Fixed smatch warning reported by kernel test robot here: https://lore.kernel.org/all/202506300509.2S1tygch-lkp@xxxxxxxxx/ Link to v5: https://lore.kernel.org/all/20250627175544.1063910-1-nilay@xxxxxxxxxxxxx/ 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@xxxxxxxxxxxxx/ 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@xxxxxxxxxxxxx/ 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@xxxxxxxxxxxxx/ 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@xxxxxxxxxxxxx/ 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(-) -- 2.50.0
