From: "Michael S. Tsirkin" <mst@redhat.com>
To: Stefano Garzarella <sgarzare@redhat.com>
Cc: virtualization@lists.linux-foundation.org,
linux-kernel@vger.kernel.org, stefanha@redhat.com,
kvm@vger.kernel.org, netdev@vger.kernel.org,
Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()
Date: Fri, 14 Jan 2022 07:45:35 -0500 [thread overview]
Message-ID: <20220114074454-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20220114090508.36416-1-sgarzare@redhat.com>
On Fri, Jan 14, 2022 at 10:05:08AM +0100, Stefano Garzarella wrote:
> In vhost_enable_notify() we enable the notifications and we read
> the avail index to check if new buffers have become available in
> the meantime.
>
> We are not caching the avail index, so when the device will call
> vhost_get_vq_desc(), it will find the old value in the cache and
> it will read the avail index again.
>
> It would be better to refresh the cache every time we read avail
> index, so let's change vhost_enable_notify() caching the value in
> `avail_idx` and compare it with `last_avail_idx` to check if there
> are new buffers available.
>
> Anyway, we don't expect a significant performance boost because
> the above path is not very common, indeed vhost_enable_notify()
> is often called with unlikely(), expecting that avail index has
> not been updated.
>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
... and can in theory even hurt due to an extra memory write.
So ... performance test restults pls?
> ---
> v1:
> - improved the commit description [MST, Jason]
> ---
> drivers/vhost/vhost.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 59edb5a1ffe2..07363dff559e 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> &vq->avail->idx, r);
> return false;
> }
> + vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>
> - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
> + return vq->avail_idx != vq->last_avail_idx;
> }
> EXPORT_SYMBOL_GPL(vhost_enable_notify);
>
> --
> 2.31.1
next prev parent reply other threads:[~2022-01-14 12:45 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-14 9:05 [PATCH v1] vhost: cache avail index in vhost_enable_notify() Stefano Garzarella
2022-01-14 12:45 ` Michael S. Tsirkin [this message]
2022-01-14 13:38 ` Stefano Garzarella
2022-01-14 13:40 ` Michael S. Tsirkin
2022-01-20 15:08 ` Stefano Garzarella
2022-01-20 16:55 ` Michael S. Tsirkin
2022-01-24 11:31 ` Stefan Hajnoczi
2022-01-25 11:14 ` Stefano Garzarella
2022-01-25 16:32 ` Stefan Hajnoczi
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