linux-kernel - Re: [PATCH 6/6] vhost_net: remove the max pending check

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Message-ID: <20130816100225.GD21821@redhat.com>
Date:	Fri, 16 Aug 2013 13:02:26 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] vhost_net: remove the max pending check

On Fri, Aug 16, 2013 at 01:16:30PM +0800, Jason Wang wrote:
> We used to limit the max pending DMAs to prevent guest from pinning too many
> pages. But this could be removed since:
> 
> - We have the sk_wmem_alloc check in both tun/macvtap to do the same work
> - This max pending check were almost useless since it was one done when there's
>   no new buffers coming from guest. Guest can easily exceeds the limitation.
> - We've already check upend_idx != done_idx and switch to non zerocopy then. So
>   even if all vq->heads were used, we can still does the packet transmission.

We can but performance will suffer.

> 
> So remove this check completely.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/vhost/net.c |   13 -------------
>  1 files changed, 0 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index a035a89..ed3f165 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -38,8 +38,6 @@ MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
>   * Using this limit prevents one virtqueue from starving others. */
>  #define VHOST_NET_WEIGHT 0x80000
>  
> -/* MAX number of TX used buffers for outstanding zerocopy */
> -#define VHOST_MAX_PEND 128
>  #define VHOST_GOODCOPY_LEN 256
>  
>  /*
> @@ -372,17 +370,6 @@ static void handle_tx(struct vhost_net *net)
>  			break;
>  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>  		if (head == vq->num) {
> -			int num_pends;
> -
> -			/* If more outstanding DMAs, queue the work.
> -			 * Handle upend_idx wrap around
> -			 */
> -			num_pends = likely(nvq->upend_idx >= nvq->done_idx) ?
> -				    (nvq->upend_idx - nvq->done_idx) :
> -				    (nvq->upend_idx + UIO_MAXIOV -
> -				     nvq->done_idx);
> -			if (unlikely(num_pends > VHOST_MAX_PEND))
> -				break;
>  			if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>  				vhost_disable_notify(&net->dev, vq);
>  				continue;
> -- 
> 1.7.1
--
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