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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: David Runge <dave@sleepmap.de>
Cc: linux-rt-users@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Daniel Wagner <dwagner@suse.de>
Subject: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT
Date: Fri, 23 Oct 2020 13:04:00 +0200	[thread overview]
Message-ID: <20201023110400.bx3uzsb7xy5jtsea@linutronix.de> (raw)
In-Reply-To: <20201021175059.GA4989@hmbx>

blk_mq_complete_request_remote() will dispatch request completion to
another CPU via IPI if the CPU belongs to a different cache domain.

This breaks on PREEMPT_RT because the IPI function will complete the
request in IRQ context which includes acquiring spinlock_t typed locks.
Completing the IPI request in softirq on the remote CPU is probably less
efficient because it would require to wake ksoftirqd for this task
(which runs at SCHED_OTHER).

Ignoring the IPI request and completing the request locally is probably
the best option. It be completed either in the IRQ-thread or at the end
of the routine in softirq context.

Let blk_mq_complete_need_ipi() return that there is no need for IPI on
PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e37aa31332b70..99d2fb51e0e84 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -647,7 +647,7 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq)
 {
 	int cpu = raw_smp_processor_id();
 
-	if (!IS_ENABLED(CONFIG_SMP) ||
+	if (!IS_ENABLED(CONFIG_SMP) || IS_ENABLED(CONFIG_PREEMPT_RT) ||
 	    !test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags))
 		return false;
 
-- 
2.28.0


       reply	other threads:[~2020-10-23 11:04 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20201021175059.GA4989@hmbx>
2020-10-23 11:04 ` Sebastian Andrzej Siewior [this message]
2020-10-23 11:21   ` [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Christoph Hellwig
2020-10-23 13:52     ` Sebastian Andrzej Siewior
2020-10-27  9:26       ` Christoph Hellwig
2020-10-27 10:11         ` Sebastian Andrzej Siewior
2020-10-27 16:07           ` Christoph Hellwig
2020-10-27 17:05             ` Thomas Gleixner
2020-10-27 17:23               ` Christoph Hellwig
2020-10-27 17:59                 ` Sebastian Andrzej Siewior
2020-10-27 20:58                 ` Sebastian Andrzej Siewior
2020-10-28  6:56                   ` Christoph Hellwig
2020-10-28 14:12                     ` [PATCH 1/3] blk-mq: Don't complete on a remote CPU in force threaded mode Sebastian Andrzej Siewior
2020-10-28 14:12                       ` [PATCH 2/3] blk-mq: Always complete remote completions requests in softirq Sebastian Andrzej Siewior
2020-10-28 14:12                       ` [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done Sebastian Andrzej Siewior
2020-10-28 14:44                         ` Christoph Hellwig
2020-10-28 14:47                           ` Sebastian Andrzej Siewior
2020-10-29 13:12                         ` Sebastian Andrzej Siewior
2020-10-29 14:05                           ` Christoph Hellwig
2020-10-29 14:56                             ` Sebastian Andrzej Siewior
2020-10-29 14:57                               ` Christoph Hellwig
2020-10-29 20:03                                 ` Sagi Grimberg
2020-10-29 21:01                                   ` Sebastian Andrzej Siewior
2020-10-29 21:07                                     ` Sagi Grimberg
2020-10-31 10:41                                       ` Sebastian Andrzej Siewior
2020-10-31 15:00                                         ` Jens Axboe
2020-10-31 15:01                                           ` Jens Axboe
2020-10-31 18:09                                             ` Christoph Hellwig
2020-11-02  9:55                                           ` Sebastian Andrzej Siewior
2020-11-02 18:12                                             ` Christoph Hellwig
2020-11-04 19:15                                               ` Sagi Grimberg
2020-11-06 15:23                                               ` Sebastian Andrzej Siewior
2020-10-28 10:04                 ` [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Peter Zijlstra

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