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From f7d87d26b4c9bfd083484542889f392867c1c85b Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 4 Dec 2020 20:13:54 +0100
Subject: [PATCH 063/323] blk-mq: Don't complete on a remote CPU in force
threaded mode
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/older/patches-5.10.204-rt100.tar.xz
With force threaded interrupts enabled, raising softirq from an SMP
function call will always result in waking the ksoftirqd thread. This is
not optimal given that the thread runs at SCHED_OTHER priority.
Completing the request in hard IRQ-context on PREEMPT_RT (which enforces
the force threaded mode) is bad because the completion handler may
acquire sleeping locks which violate the locking context.
Disable request completing on a remote CPU in force threaded mode.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
block/blk-mq.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index e153a36c9ba3..cc41373cf346 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -652,6 +652,14 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq)
if (!IS_ENABLED(CONFIG_SMP) ||
!test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags))
return false;
+ /*
+ * With force threaded interrupts enabled, raising softirq from an SMP
+ * function call will always result in waking the ksoftirqd thread.
+ * This is probably worse than completing the request on a different
+ * cache domain.
+ */
+ if (force_irqthreads)
+ return false;
/* same CPU or cache domain? Complete locally */
if (cpu == rq->mq_ctx->cpu ||
--
2.43.0
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