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

[Ionut Nechita (WindRiver)]

From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>

Hi Ming,

Thank you for the feedback!

You're absolutely right - blk_mq_cpuhp_lock is only acquired in the slow
path (setup/cleanup operations during queue initialization/teardown), not
in the fast I/O path.

Looking at my testing results more carefully:
- The queue_lock patch (PATCH 1/2) alone restores performance to 640 MB/s
- The cpuhp_lock conversion (PATCH 2/2) doesn't contribute to fixing the
  I/O regression

The cpuhp_lock is used in:
- blk_mq_remove_cpuhp() - queue cleanup
- blk_mq_add_hw_queues_cpuhp() - queue setup
- blk_mq_remove_hw_queues_cpuhp() - queue cleanup

These are indeed slow path operations with no contention in the I/O hot
path.

I'll drop the second patch (cpuhp_lock conversion) and send v2 with only
the queue_lock fix, which addresses the actual bottleneck: removing the
sleeping lock from blk_mq_run_hw_queue() that was causing IRQ threads to
serialize and enter D-state during I/O completion.

Best regards,
Ionut