Re: [PATCH net] virtio-net: add cond_resched() to the command waiting loop

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, edumazet@google.com,
	kuba@kernel.org, gautam.dawar@xilinx.com, pabeni@redhat.com,
	davem@davemloft.net
Subject: Re: [PATCH net] virtio-net: add cond_resched() to the command waiting loop
Date: Mon, 5 Sep 2022 03:15:24 -0400	[thread overview]
Message-ID: <20220905031405-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20220905045341.66191-1-jasowang@redhat.com>

On Mon, Sep 05, 2022 at 12:53:41PM +0800, Jason Wang wrote:
> Adding cond_resched() to the command waiting loop for a better
> co-operation with the scheduler. This allows to give CPU a breath to
> run other task(workqueue) instead of busy looping when preemption is
> not allowed.
> 
> What's more important. This is a must for some vDPA parent to work
> since control virtqueue is emulated via a workqueue for those parents.
> 
> Fixes: bda324fd037a ("vdpasim: control virtqueue support")

That's a weird commit to fix. so it fixes the simulator?

> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/net/virtio_net.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index ece00b84e3a7..169368365d6a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2000,8 +2000,10 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
>  	 * into the hypervisor, so the request should be handled immediately.
>  	 */
>  	while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> -	       !virtqueue_is_broken(vi->cvq))
> +	       !virtqueue_is_broken(vi->cvq)) {
> +		cond_resched();
>  		cpu_relax();
> +	}

with cond_resched do we still need cpu_relax?

>  	return vi->ctrl->status == VIRTIO_NET_OK;
>  }
> -- 
> 2.25.1

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