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@xxxxxxxxxxxxx/#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
