linux-kernel - Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and ioq0

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Message-ID: <CAHp75Vet0gt0RW4eu+TCE8qBSk2ymMKkG5=SuM9ytzO3-6Y+Mg@mail.gmail.com>
Date:   Wed, 28 Feb 2018 17:59:54 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] nvme-pci: assign separate irq vectors for adminq and ioq0

On Wed, Feb 28, 2018 at 5:48 PM, Jianchao Wang
<jianchao.w.wang@...cle.com> 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. It
>    just has only one hw queue, it is unreasonable to do this.
>  - For ioq0, when the irq fires for io completion, the adminq irq
>    action on this irq vector will introduce an uncached access on
>    adminq cqe at least, even worse when adminq is busy.
>
> To improve this, allocate separate irq vectors for adminq and
> ioq0, and not set irq affinity for adminq one. If just one irq
> vector, setup adminq + 1 ioq and let them share it. In addition
> add new helper interface nvme_ioq_vector to get ioq vector.

> +static inline unsigned int nvme_ioq_vector(struct nvme_dev *dev,
> +               unsigned int qid)
> +{
> +       /*
> +        * If controller has only legacy or single-message MSI, there will
> +        * be only 1 irq vector. At the moment, we setup adminq + 1 ioq
> +        * and let them share irq vector.
> +        */
> +       return (dev->num_vecs == 1) ? 0 : qid;

Redundant parens.

> +}

>
>         for (i = dev->ctrl.queue_count; i <= dev->max_qid; i++) {
> -               /* vector == qid - 1, match nvme_create_queue */

>                 if (nvme_alloc_queue(dev, i, dev->q_depth,
> -                    pci_irq_get_node(to_pci_dev(dev->dev), i - 1))) {
> +                    pci_irq_get_node(to_pci_dev(dev->dev),
> +                                nvme_ioq_vector(dev, i)))) {

Perhaps better to introduce a temporary variable to make it readable?

>                         ret = -ENOMEM;
>                         break;
>                 }

> +       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->num_vecs = ret;
> +       dev->max_qid = (ret > 1) ? (ret - 1) : 1;

I don not see how ret can possible be < 1 here.

Thus, the logic looks like this:
if ret >= 2 => return ret - 1; // Possible variants [1..ret - 1]
if ret == 1 => return 1;

So, for ret == 1 or ret == 2 we still use 1.

Is it by design?

Can it be written like

dev->max_qid = max(ret - 1, 1);

-- 
With Best Regards,
Andy Shevchenko