block/blk-mq: fix RT kernel issues and interrupt context warnings #433
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added 2 commits
January 9, 2026 14:01
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: [email protected]
Signed-off-by: Ionut Nechita <[email protected]>
…context Fix warning "WARN_ON_ONCE(!async && in_interrupt())" that occurs during SCSI device scanning when blk_freeze_queue_start() calls blk_mq_run_hw_queues() synchronously from interrupt context. The issue happens during device removal/scanning when: 1. blk_mq_destroy_queue() -> blk_queue_start_drain() 2. blk_freeze_queue_start() calls blk_mq_run_hw_queues(q, false) 3. This triggers the warning in blk_mq_run_hw_queue() when in interrupt context Change the synchronous call to asynchronous to avoid running in interrupt context. Fixes: Warning in blk_mq_run_hw_queue+0x1fa/0x260 Signed-off-by: Ionut Nechita <[email protected]>
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Pull request for series with
subject: block/blk-mq: fix RT kernel issues and interrupt context warnings
version: 2
url: https://patchwork.kernel.org/project/linux-block/list/?series=1035804