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

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

On 8/6/25 6:58 AM, Jens Axboe wrote:
> On 8/4/25 10:58 PM, 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.
>>
>> 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?
> 
> Please heed Ming's advice, moving this from a static key to an atomic
> queue flags ops is pointless, may as well kill it at that point.
> 
Yes I agree and personally I like static key very much as it's lightweight. 
And I also liked the way you used it in IO hotpath so that we avoid cost of
fetching q->rq_qos when not needed. 
Having said that, I also tried Ming's suggestion but that didn't work out
due to the fact that "cpu_hotplug_lock is widely used across various kernel
subsystems— not just in CPU hotplug-specific paths. There are several code
paths outside of the hotplug core that acquire cpu_hotplug_lock and subsequently
perform memory allocations using GFP_KERNEL". So essentially adopting to use
GFP_NOIO in cpu hotplug code may not help. You might have missed my reply to
Ming's suggestion, you may refer it here:
https://lore.kernel.org/all/897eaaa4-31c7-4661-b5d4-3e2bef1fca1e@linux.ibm.com/#t

> I see v2 is out now with the exact same approach.
> 
Yes I sent out v2 just for fixing minor things in the original patch as I
outlined it in the v2 changelog.


Thanks,
--Nilay