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

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Date:   Mon, 25 Jan 2021 08:10:16 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/3] blk-mq: Always complete remote completions requests
 in softirq

On 1/23/21 9:10 PM, Sebastian Andrzej Siewior wrote:
> 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@...utronix.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@...e.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
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