linux-kernel - Re: [PATCH] nvme-pci: assign separate irq vectors for adminq and ioq0

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Date:   Tue, 27 Feb 2018 08:13:11 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     axboe@...com, hch@....de, sagi@...mberg.me,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-pci: assign separate irq vectors for adminq and ioq0

On Tue, Feb 27, 2018 at 04:46:17PM +0800, Jianchao Wang wrote:
> Currently, adminq and ioq0 share the same irq vector. This is
> unfair for both amdinq and ioq0.
>  - For adminq, its completion irq has to be bound on cpu0.
>  - For ioq0, when the irq fires for io completion, the adminq irq
>    action has to be checked also.

This change log could use some improvements. Why is it bad if admin
interrupts affinity is with cpu0?

Are you able to measure _any_ performance difference on IO queue 1 vs IO
queue 2 that you can attribute to IO queue 1's sharing vector 0?
 
> @@ -1945,11 +1947,11 @@ static int nvme_setup_io_queues(struct nvme_dev *dev)
>  	 * setting up the full range we need.
>  	 */
>  	pci_free_irq_vectors(pdev);
> -	nr_io_queues = pci_alloc_irq_vectors(pdev, 1, nr_io_queues,
> -			PCI_IRQ_ALL_TYPES | PCI_IRQ_AFFINITY);
> -	if (nr_io_queues <= 0)
> +	ret = pci_alloc_irq_vectors_affinity(pdev, 1, (nr_io_queues + 1),
> +			PCI_IRQ_ALL_TYPES | PCI_IRQ_AFFINITY, &affd);
> +	if (ret <= 0)
>  		return -EIO;
> -	dev->max_qid = nr_io_queues;
> +	dev->max_qid = ret - 1;

So controllers that have only legacy or single-message MSI don't get any
IO queues?