Re: [PATCH 0/2] block: blk-rq-qos: replace static key with atomic bitop

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 04, 2025 at 05:51:09PM +0530, Nilay Shroff wrote:
> This patchset replaces the use of a static key in the I/O path (rq_qos_
> xxx()) with an atomic queue flag (QUEUE_FLAG_QOS_ENABLED). This change
> is made to eliminate a potential deadlock introduced by the use of static
> keys in the blk-rq-qos infrastructure, as reported by lockdep during 
> blktests block/005[1].
> 
> The original static key approach was introduced to avoid unnecessary
> dereferencing of q->rq_qos when no blk-rq-qos module (e.g., blk-wbt or
> blk-iolatency) is configured. While efficient, enabling a static key at
> runtime requires taking cpu_hotplug_lock and jump_label_mutex, which 
> becomes problematic if the queue is already frozen — causing a reverse
> dependency on ->freeze_lock. This results in a lockdep splat indicating
> a potential deadlock.
> 
> To resolve this, we now gate q->rq_qos access with a q->queue_flags
> bitop (QUEUE_FLAG_QOS_ENABLED), avoiding the static key and the associated
> locking altogether.
> 
> I compared both static key and atomic bitop implementations using ftrace
> function graph tracer over ~50 invocations of rq_qos_issue() while ensuring
> blk-wbt/blk-iolatency were disabled (i.e., no QoS functionality). For
> easy comparision, I made rq_qos_issue() noinline. The comparision was
> made on PowerPC machine.
> 
> Static Key (disabled : QoS is not configured):
> 5d0: 00 00 00 60     nop    # patched in by static key framework (not taken)
> 5d4: 20 00 80 4e     blr    # return (branch to link register)
> 
> Only a nop and blr (branch to link register) are executed — very lightweight.
> 
> atomic bitop (QoS is not configured):
> 5d0: 20 00 23 e9     ld      r9,32(r3)     # load q->queue_flags
> 5d4: 00 80 29 71     andi.   r9,r9,32768   # check QUEUE_FLAG_QOS_ENABLED (bit 15)
> 5d8: 20 00 82 4d     beqlr                 # return if bit not set
> 
> This performs an ld and and andi. before returning. Slightly more work, 
> but q->queue_flags is typically hot in cache during I/O submission.
> 
> With Static Key (disabled):
> Duration (us): min=0.668 max=0.816 avg≈0.750
> 
> With atomic bitop QUEUE_FLAG_QOS_ENABLED (bit not set):
> Duration (us): min=0.684 max=0.834 avg≈0.759
> 
> As expected, both versions are almost similar in cost. The added latency
> from an extra ld and andi. is in the range of ~9ns.
> 
> There're two patches in the series. The first patch replaces static key
> with QUEUE_FLAG_QOS_ENABLED. The second patch ensures that we disable
> the QUEUE_FLAG_QOS_ENABLED when the queue no longer has any associated
> rq_qos policies.
> 
> As usual, feedback and review comments are welcome!
> 
> [1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/


Another approach is to call memalloc_noio_save() in cpu hotplug code...


Thanks,
Ming