Re: [PATCH 2/2] block/blk-mq: convert blk_mq_cpuhp_lock to raw_spinlock for RT

[Ming Lei <ming.lei@redhat.com>]

On Sat, Dec 20, 2025 at 01:02:41PM +0200, Ionut Nechita (WindRiver) wrote:
> From: Ionut Nechita <ionut.nechita@windriver.com>
> 
> Commit 58bf93580fec ("blk-mq: move cpuhp callback registering out of
> q->sysfs_lock") introduced a global mutex blk_mq_cpuhp_lock to avoid
> lockdep warnings between sysfs_lock and CPU hotplug lock.
> 
> On RT kernels (CONFIG_PREEMPT_RT), regular mutexes are converted to
> rt_mutex (sleeping locks). When block layer operations need to acquire
> blk_mq_cpuhp_lock, IRQ threads processing I/O completions may sleep,
> causing additional contention on top of the queue_lock issue from
> commit 679b1874eba7 ("block: fix ordering between checking
> QUEUE_FLAG_QUIESCED request adding").
> 
> Test case (MegaRAID 12GSAS with 8 MSI-X vectors on RT kernel):
> - v6.6.68-rt with queue_lock fix: 640 MB/s (queue_lock fixed)
> - v6.6.69-rt: still exhibits contention due to cpuhp_lock mutex
> 
> The functions protected by blk_mq_cpuhp_lock only perform fast,
> non-sleeping operations:
> - hlist_unhashed() checks
> - cpuhp_state_add_instance_nocalls() - just hlist manipulation
> - cpuhp_state_remove_instance_nocalls() - just hlist manipulation
> - INIT_HLIST_NODE() initialization
> 
> The _nocalls variants do not invoke state callbacks and only manipulate
> data structures, making them safe to call under raw_spinlock.
> 
> Convert blk_mq_cpuhp_lock from mutex to raw_spinlock to prevent it from
> becoming a sleeping lock in RT kernel. This eliminates the contention
> bottleneck while maintaining the lockdep fix's original intent.

What is the contention bottleneck? blk_mq_cpuhp_lock is only acquired in
slow code path, and it isn't required in fast io path.

> 
> Fixes: 58bf93580fec ("blk-mq: move cpuhp callback registering out of q->sysfs_lock")

With the 1st patch, the perf becomes 640MB/s, same with before regression.

So can you share what you try to fix with this patch?

> Cc: stable@vger.kernel.org
> Signed-off-by: Ionut Nechita <ionut.nechita@windriver.com>
> ---
>  block/blk-mq.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 5fb8da4958d0..3982e24b1081 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -43,7 +43,7 @@
>  
>  static DEFINE_PER_CPU(struct llist_head, blk_cpu_done);
>  static DEFINE_PER_CPU(call_single_data_t, blk_cpu_csd);
> -static DEFINE_MUTEX(blk_mq_cpuhp_lock);
> +static DEFINE_RAW_SPINLOCK(blk_mq_cpuhp_lock);
>  
>  static void blk_mq_insert_request(struct request *rq, blk_insert_t flags);
>  static void blk_mq_request_bypass_insert(struct request *rq,
> @@ -3641,9 +3641,9 @@ static void __blk_mq_remove_cpuhp(struct blk_mq_hw_ctx *hctx)
>  
>  static void blk_mq_remove_cpuhp(struct blk_mq_hw_ctx *hctx)
>  {
> -	mutex_lock(&blk_mq_cpuhp_lock);
> +	raw_spin_lock(&blk_mq_cpuhp_lock);
>  	__blk_mq_remove_cpuhp(hctx);
> -	mutex_unlock(&blk_mq_cpuhp_lock);
> +	raw_spin_unlock(&blk_mq_cpuhp_lock);

__blk_mq_remove_cpuhp()
	->cpuhp_state_remove_instance_nocalls()
		->__cpuhp_state_remove_instance
			->cpus_read_lock
				->percpu_down_read
					->percpu_down_read_internal
						->might_sleep()


Thanks,
Ming