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blk-mq: Don't complete on a remote CPU in force threaded mode

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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>
This commit is contained in:
Sebastian Andrzej Siewior 2020-12-04 20:13:54 +01:00 committed by Jens Axboe
parent 53a7f65583
commit 71425189b2
1 changed files with 8 additions and 0 deletions
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8
block/blk-mq.c
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@ -650,6 +650,14 @@ 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 ||