linux-kernel - Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT

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Date:   Tue, 27 Oct 2020 17:23:09 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        David Runge <dave@...epmap.de>, linux-rt-users@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <dwagner@...e.de>
Subject: Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT

On Tue, Oct 27, 2020 at 06:05:15PM +0100, Thomas Gleixner wrote:
> > Is there a way to raise a softirq and preferably place it on a given
> > CPU without our IPI dance?  That should be a win-win situation for
> > everyone.
> 
> Not really. Softirq pending bits are strictly per cpu and we don't have
> locking or atomics to set them remotely. Even if we had that, then you'd
> still need a mechanism to make sure that the remote CPU actually
> processes them. So you'd still need an IPI of some sorts.

Ok.  I was hoping we could hide this in core code somehow, especially
a peterz didn't like the use of smp_call_function_single_async in the
blk-mq completion code very much.

Sebastian, would this solve your preempt-rt and lockdep issues?


diff --git a/block/blk-mq.c b/block/blk-mq.c
index cdced4aca2e812..5c125fb11b5691 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -626,19 +626,7 @@ 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)