On 2025/12/23 04:15, Ionut Nechita (WindRiver) wrote:
From: Ionut Nechita ionut.nechita@windriver.com
Commit 679b1874eba7 ("block: fix ordering between checking QUEUE_FLAG_QUIESCED request adding") introduced queue_lock acquisition in blk_mq_run_hw_queue() to synchronize QUEUE_FLAG_QUIESCED checks.
On RT kernels (CONFIG_PREEMPT_RT), regular spinlocks are converted to rt_mutex (sleeping locks). When multiple MSI-X IRQ threads process I/O completions concurrently, they contend on queue_lock in the hot path, causing all IRQ threads to enter D (uninterruptible sleep) state. This serializes interrupt processing completely.
Test case (MegaRAID 12GSAS with 8 MSI-X vectors on RT kernel):
- Good (v6.6.52-rt): 640 MB/s sequential read
- Bad (v6.6.64-rt): 153 MB/s sequential read (-76% regression)
- 6-8 out of 8 MSI-X IRQ threads stuck in D-state waiting on queue_lock
The original commit message mentioned memory barriers as an alternative approach. Use full memory barriers (smp_mb) instead of queue_lock to provide the same ordering guarantees without sleeping in RT kernel.
Memory barriers ensure proper synchronization:
- CPU0 either sees QUEUE_FLAG_QUIESCED cleared, OR
- CPU1 sees dispatch list/sw queue bitmap updates
This maintains correctness while avoiding lock contention that causes RT kernel IRQ threads to sleep in the I/O completion path.
Fixes: 679b1874eba7 ("block: fix ordering between checking QUEUE_FLAG_QUIESCED request adding") Cc: stable@vger.kernel.org Signed-off-by: Ionut Nechita ionut.nechita@windriver.com
block/blk-mq.c | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c index 5da948b07058..5fb8da4958d0 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2292,22 +2292,19 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async) might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING);
- /*
* First lockless check to avoid unnecessary overhead.* Memory barrier below synchronizes with blk_mq_unquiesce_queue().*/
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);
/* Synchronize with blk_mq_unquiesce_queue() */
Memory barriers must be used in pairs. So how to synchronize?
smp_mb();
spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);- if (!need_run) return;
/* Ensure dispatch list/sw queue updates visible before execution */smp_mb();
Why we need another barrier? What order does this barrier guarantee?
Thanks.
} if (async || !cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask)) {