Hi,
在 2025/08/06 9:28, Jens Axboe 写道:
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.
Nilay already tested and replied this is a dead end :(
I don't quite understand why it's pointless, if rq_qos is never enabled,
an atmoic queue_flag is still minor optimization, isn't it?
Thanks,
Kuai
I see v2 is out now with the exact same approach.
