[PATCH 0/2] block/blk-mq: fix RT kernel performance regressions

[Ionut Nechita (WindRiver)]

From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>

This series addresses two critical performance regressions in the block
layer multiqueue (blk-mq) subsystem when running on PREEMPT_RT kernels.

On RT kernels, regular spinlocks are converted to sleeping rt_mutex locks,
which can cause severe performance degradation in the I/O hot path. This
series converts two problematic locking patterns to prevent IRQ threads
from sleeping during I/O operations.

Testing on MegaRAID 12GSAS controller with 8 MSI-X vectors shows:
- v6.6.52-rt (before regression): 640 MB/s sequential read
- v6.6.64-rt (regression introduced): 153 MB/s (-76% regression)
- v6.6.68-rt with queue_lock fix only: 640 MB/s (performance restored)
- v6.6.69-rt with both fixes: expected similar or better performance

The first patch replaces queue_lock with memory barriers in the I/O
completion hot path, eliminating the contention that caused IRQ threads
to sleep. The second patch converts the global blk_mq_cpuhp_lock from
mutex to raw_spinlock to prevent sleeping during CPU hotplug operations.

Both conversions are safe because the protected code paths only perform
fast, non-blocking operations (memory barriers, list/hlist manipulation,
flag checks).

Ionut Nechita (2):
  block/blk-mq: fix RT kernel regression with queue_lock in hot path
  block/blk-mq: convert blk_mq_cpuhp_lock to raw_spinlock for RT

 block/blk-mq.c | 33 +++++++++++++++------------------
 1 file changed, 15 insertions(+), 18 deletions(-)

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2.52.0