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
