Thread (23 messages) 23 messages, 6 authors, 2021-02-10

Re: [PATCH 2/3] blk-mq: Always complete remote completions requests in softirq

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From: Hannes Reinecke <hare@suse.de>
Date: 2021-01-25 08:39:36
Also in: lkml

On 1/23/21 9:10 PM, Sebastian Andrzej Siewior wrote:
quoted hunk ↗ jump to hunk
Controllers with multiple queues have their IRQ-handelers pinned to a
CPU. The core shouldn't need to complete the request on a remote CPU.

Remove this case and always raise the softirq to complete the request.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
  block/blk-mq.c | 14 +-------------
  1 file changed, 1 insertion(+), 13 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index f285a9123a8b0..90348ae518461 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -628,19 +628,7 @@ static void __blk_mq_complete_request_remote(void *data)
  {
  	struct request *rq = data;
  
-	/*
-	 * For most of single queue controllers, there is only one irq vector
-	 * for handling I/O completion, and the only irq's affinity is set
-	 * to all possible CPUs.  On most of ARCHs, this affinity means the irq
-	 * is handled on one specific CPU.
-	 *
-	 * So complete I/O requests in softirq context in case of single queue
-	 * devices to avoid degrading I/O performance due to irqsoff latency.
-	 */
-	if (rq->q->nr_hw_queues == 1)
-		blk_mq_trigger_softirq(rq);
-	else
-		rq->q->mq_ops->complete(rq);
+	blk_mq_trigger_softirq(rq);
  }
  
  static inline bool blk_mq_complete_need_ipi(struct request *rq)
I don't get this.
This code is about _avoiding_ having to raise a softirq if the driver exports more than one hardware queue.
So where exactly does the remote CPU case come in here?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer
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