linux-kernel - Re: [PATCH 2/2] virtio_ring: add unlikely annotation for free descs check

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Message-ID: <CACGkMEsVjcQGLr8=OwpbhSJY3_Nn_am_YK7gQKdvV6PKUb9qRw@mail.gmail.com>
Date:   Tue, 29 Mar 2022 12:57:59 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xianting Tian <xianting.tian@...ux.alibaba.com>
Cc:     mst <mst@...hat.com>, Stefano Garzarella <sgarzare@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] virtio_ring: add unlikely annotation for free descs check

On Mon, Mar 28, 2022 at 6:58 PM Xianting Tian
<xianting.tian@...ux.alibaba.com> wrote:
>
> The 'if (vq->vq.num_free < descs_used)' check will almost always be false.
>
> Signed-off-by: Xianting Tian <xianting.tian@...ux.alibaba.com>

Acked-by: Jason Wang <jasowang@...hat.com>

> ---
>  drivers/virtio/virtio_ring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index d597fc0874ec..ab6d5f0cb579 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -525,7 +525,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>                 descs_used = total_sg;
>         }
>
> -       if (vq->vq.num_free < descs_used) {
> +       if (unlikely(vq->vq.num_free < descs_used)) {
>                 pr_debug("Can't add buf len %i - avail = %i\n",
>                          descs_used, vq->vq.num_free);
>                 /* FIXME: for historical reasons, we force a notify here if
> --
> 2.17.1
>