Re: [PATCHv8 0/3] block: move sched_tags allocation/de-allocation outside of locking context

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

Hi Jens,

I believe this patchset may have fallen through the cracks. I've just rebased it on
top of the block-6.17 branch at commit 5421681bc3ef ("blk-ioc: don't hold queue_lock
for ioc_lookup_icq()") and sent out v8.

Could you please consider pulling it for Linux 6.17?

Thanks!

--Nilay

On 7/30/25 1:16 PM, Nilay Shroff wrote:
> 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 v7:
>     - Rebased code on top of block-6.17, commit 5421681bc3ef ("blk-ioc: don't
>       hold queue_lock for ioc_lookup_icq()")
> Link to v7: https://lore.kernel.org/all/20250701081954.57381-1-nilay@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           |   4 +-
>  block/elevator.c      |  38 +++++--
>  block/elevator.h      |  16 ++-
>  block/kyber-iosched.c |  11 +--
>  block/mq-deadline.c   |  14 +--
>  9 files changed, 228 insertions(+), 119 deletions(-)
>