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
