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

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

On Tue, Aug 05, 2025 at 10:28:14AM +0530, Nilay Shroff wrote:
> 
> 
> On 8/4/25 7:12 PM, Ming Lei wrote:
> > 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...
> > 
> Yes that would help fix this. However per the general usage of GFP_NOIO scope in 
> kernel, it is used when we're performing memory allocations in a context where I/O
> must not be initiated, because doing so could cause deadlocks or recursion. 
> 
> So we typically, use GFP_NOIO in a code path that is already doing I/O, such as:
> - In block layer context: during request submission 
> - Filesystem writeback, or swap-out.
> - Memory reclaim or writeback triggered by memory pressure.

If you grep blk_mq_freeze_queue, you will see the above list is far from
enough, :-)

> 
> The cpu hotplug code may not be running in any of the above context. So
> IMO, adding memalloc_noio_save() in the cpu hotplug code would not be 
> a good idea, isn't it?

The reasoning(A -> B) looks correct, but the condition A is obviously not.


Thanks,
Ming