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

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

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.

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?

Thanks,
--Nilay