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
