Re: [PATCH v3 1/1] block/blk-mq: use atomic_t for quiesce_depth to avoid lock contention on RT

[Hillf Danton]

On Wed, 11 Feb 2026 22:39:29 +0200
> From: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>
> 
> In RT kernel (PREEMPT_RT), commit 679b1874eba7 ("block: fix ordering
> between checking QUEUE_FLAG_QUIESCED request adding") causes severe
> performance regression on systems with multiple MSI-X interrupt
> vectors.
> 
> The above change introduced spinlock_t queue_lock usage in
> blk_mq_run_hw_queue() to synchronize QUEUE_FLAG_QUIESCED checks
> with blk_mq_unquiesce_queue(). While this works correctly in
> standard kernel, it causes catastrophic serialization in RT kernel
> where spinlock_t converts to sleeping rt_mutex.
> 
> Problem in RT kernel:
> - blk_mq_run_hw_queue() is called from IRQ thread context
> - With multiple MSI-X vectors, all IRQ threads contend on
>   the same queue_lock
> - queue_lock becomes rt_mutex (sleeping) in RT kernel
> - IRQ threads serialize and enter D-state waiting for lock
> - Throughput drops from 640 MB/s to 153 MB/s
> 
> Solution:
> Convert quiesce_depth to atomic_t and use it directly for quiesce
> state checking, eliminating QUEUE_FLAG_QUIESCED entirely. This
> removes the need for any locking in the hot path.
> 
> The atomic counter serves as both the depth tracker and the quiesce
> indicator (depth > 0 means quiesced). This eliminates the race
> window that existed between updating the depth and the flag.
> 
> Memory ordering is ensured by:
> - smp_mb__after_atomic() after modifying quiesce_depth
> - smp_rmb() before re-checking quiesce state in
>   blk_mq_run_hw_queue()
> 
> Performance impact:
> - RT kernel: eliminates lock contention, restores full throughput
> - Non-RT kernel: atomic ops are similar cost to the previous
>   spinlock acquire/release, no regression expected
> 
> Test results on RT kernel:
> Hardware: Broadcom/LSI MegaRAID 12GSAS/PCIe Secure SAS39xx
>   (megaraid_sas driver, 128 MSI-X vectors, 120 hw queues)
> - Before: 153 MB/s, IRQ threads in D-state
> - After:  640 MB/s, no IRQ threads blocked
> 
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Fixes: 679b1874eba7 ("block: fix ordering between checking QUEUE_FLAG_QUIESCED request adding")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Ionut Nechita <ionut.nechita@xxxxxxxxxxxxx>
> ---
>  block/blk-core.c       |  1 +
>  block/blk-mq-debugfs.c |  1 -
>  block/blk-mq.c         | 45 ++++++++++++++++--------------------------
>  include/linux/blkdev.h |  9 ++++++---
>  4 files changed, 24 insertions(+), 32 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 8387fe50ea15..4eea19426cc6 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -434,6 +434,7 @@ struct request_queue *blk_alloc_queue(struct queue_limits *lim, int node_id)
>  	mutex_init(&q->limits_lock);
>  	mutex_init(&q->rq_qos_mutex);
>  	spin_lock_init(&q->queue_lock);
> +	atomic_set(&q->quiesce_depth, 0);
>  
>  	init_waitqueue_head(&q->mq_freeze_wq);
>  	mutex_init(&q->mq_freeze_lock);
> diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> index 4896525b1c05..c63fe8864248 100644
> --- a/block/blk-mq-debugfs.c
> +++ b/block/blk-mq-debugfs.c
> @@ -89,7 +89,6 @@ static const char *const blk_queue_flag_name[] = {
>  	QUEUE_FLAG_NAME(INIT_DONE),
>  	QUEUE_FLAG_NAME(STATS),
>  	QUEUE_FLAG_NAME(REGISTERED),
> -	QUEUE_FLAG_NAME(QUIESCED),
>  	QUEUE_FLAG_NAME(RQ_ALLOC_TIME),
>  	QUEUE_FLAG_NAME(HCTX_ACTIVE),
>  	QUEUE_FLAG_NAME(SQ_SCHED),
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 968699277c3d..1e0f5a311bef 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -260,12 +260,12 @@ EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue_non_owner);
>   */
>  void blk_mq_quiesce_queue_nowait(struct request_queue *q)
>  {
> -	unsigned long flags;
> -
> -	spin_lock_irqsave(&q->queue_lock, flags);
> -	if (!q->quiesce_depth++)
> -		blk_queue_flag_set(QUEUE_FLAG_QUIESCED, q);
> -	spin_unlock_irqrestore(&q->queue_lock, flags);
> +	atomic_inc(&q->quiesce_depth);
> +	/*
> +	 * Ensure the store to quiesce_depth is visible before any
> +	 * subsequent loads in blk_mq_run_hw_queue().
> +	 */
> +	smp_mb__after_atomic();

Nope mb is not enough.

	int qd = atomic_read(&q->quiesce_depth);
	for (;;) {
		int v = qd + 1;
		if (atomic_try_cmpxchg(&q->quiesce_depth, &qd, v))
			break;
	}

More important however, why not send this patch to the RT tree instead, given
the spin lock is good and correct in mainline. By good I mean fixing RT stuff
in the mainline is not encouraged.

>  }
>  EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
>  
> @@ -314,21 +314,18 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
>   */
>  void blk_mq_unquiesce_queue(struct request_queue *q)
>  {
> -	unsigned long flags;
> -	bool run_queue = false;
> +	int depth;
>  
> -	spin_lock_irqsave(&q->queue_lock, flags);
> -	if (WARN_ON_ONCE(q->quiesce_depth <= 0)) {
> -		;
> -	} else if (!--q->quiesce_depth) {
> -		blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
> -		run_queue = true;
> -	}
> -	spin_unlock_irqrestore(&q->queue_lock, flags);
> +	depth = atomic_dec_if_positive(&q->quiesce_depth);
> +	if (WARN_ON_ONCE(depth < 0))
> +		return;
>  
> -	/* dispatch requests which are inserted during quiescing */
> -	if (run_queue)
> +	if (depth == 0) {
> +		/* Ensure the decrement is visible before running queues */
> +		smp_mb__after_atomic();
> +		/* dispatch requests which are inserted during quiescing */
>  		blk_mq_run_hw_queues(q, true);
> +	}
>  }
>  EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
>  
> @@ -2352,17 +2349,9 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
>  
>  	need_run = blk_mq_hw_queue_need_run(hctx);
>  	if (!need_run) {
> -		unsigned long flags;
> -
> -		/*
> -		 * Synchronize with blk_mq_unquiesce_queue(), because we check
> -		 * if hw queue is quiesced locklessly above, we need the use
> -		 * ->queue_lock to make sure we see the up-to-date status to
> -		 * not miss rerunning the hw queue.
> -		 */
> -		spin_lock_irqsave(&hctx->queue->queue_lock, flags);
> +		/* Pairs with smp_mb__after_atomic() in blk_mq_unquiesce_queue() */
> +		smp_rmb();
>  		need_run = blk_mq_hw_queue_need_run(hctx);
> -		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);
>  
>  		if (!need_run)
>  			return;
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 72e34acd439c..9ad725af81f6 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -516,7 +516,8 @@ struct request_queue {
>  
>  	spinlock_t		queue_lock;
>  
> -	int			quiesce_depth;
> +	/* Atomic quiesce depth - also serves as quiesced indicator (depth > 0) */
> +	atomic_t		quiesce_depth;
>  
>  	struct gendisk		*disk;
>  
> @@ -660,7 +661,6 @@ enum {
>  	QUEUE_FLAG_INIT_DONE,		/* queue is initialized */
>  	QUEUE_FLAG_STATS,		/* track IO start and completion times */
>  	QUEUE_FLAG_REGISTERED,		/* queue has been registered to a disk */
> -	QUEUE_FLAG_QUIESCED,		/* queue has been quiesced */
>  	QUEUE_FLAG_RQ_ALLOC_TIME,	/* record rq->alloc_time_ns */
>  	QUEUE_FLAG_HCTX_ACTIVE,		/* at least one blk-mq hctx is active */
>  	QUEUE_FLAG_SQ_SCHED,		/* single queue style io dispatch */
> @@ -697,7 +697,10 @@ void blk_queue_flag_clear(unsigned int flag, struct request_queue *q);
>  #define blk_noretry_request(rq) \
>  	((rq)->cmd_flags & (REQ_FAILFAST_DEV|REQ_FAILFAST_TRANSPORT| \
>  			     REQ_FAILFAST_DRIVER))
> -#define blk_queue_quiesced(q)	test_bit(QUEUE_FLAG_QUIESCED, &(q)->queue_flags)
> +static inline bool blk_queue_quiesced(struct request_queue *q)
> +{
> +	return atomic_read(&q->quiesce_depth) > 0;
> +}
>  #define blk_queue_pm_only(q)	atomic_read(&(q)->pm_only)
>  #define blk_queue_registered(q)	test_bit(QUEUE_FLAG_REGISTERED, &(q)->queue_flags)
>  #define blk_queue_sq_sched(q)	test_bit(QUEUE_FLAG_SQ_SCHED, &(q)->queue_flags)
> -- 
> 2.53.0