netdev - RE: [PATCH net v2] xen-netback: fix abuse of napi budget

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Message-ID: <9AAE0902D5BC7E449B7C8E4E778ABCD01ABE30@AMSPEX01CL01.citrite.net>
Date:	Tue, 10 Dec 2013 11:32:01 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Paul Durrant <Paul.Durrant@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: RE: [PATCH net v2] xen-netback: fix abuse of napi budget

> -----Original Message-----
> From: Paul Durrant [mailto:paul.durrant@...rix.com]
> Sent: 10 December 2013 11:28
> To: xen-devel@...ts.xen.org; netdev@...r.kernel.org
> Cc: Paul Durrant; Wei Liu; Ian Campbell; David Vrabel
> Subject: [PATCH net v2] xen-netback: fix abuse of napi budget
> 
> netback seemed to be somewhat confused about the napi budget
> parameter. The
> parameter is supposed to limit the number of skgs process in each poll, but
> netback had this confused with grant operations.
> 
> This patch fixes that, properly limiting the work done in each poll. Note
> that this limit makes sure we do not process any more data from the shared
> ring than we intend to pass back from the poll. This is important to
> prevent tx_queue potentially growing without bound.
> 
> This patch also changes the RING_FINAL_CHECK_FOR_REQUESTS in
> xenvif_build_tx_gops to a check for RING_HAS_UNCONSUMED_REQUESTS
> as the
> former call has the side effect of advancing the ring event pointer and
> therefore inviting another interrupt from the frontend before the napi
> poll has actually finished, thereby defeating the point of napi.
> 
> Signed-off-by: Paul Durrant <paul.durrant@...rix.com>

Sorry, this patch is incomplete. I need to remove the decrement of the work_to_do variable in xenvif_tx_build_gops too.

  Paul

> Cc: Wei Liu <wei.liu2@...rix.com>
> Cc: Ian Campbell <ian.campbell@...rix.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> ---
> v2:
> - More elaborate description of the problem and the fix in the comment.
> - Modified the check in xenvif_build_tx_gops.
> 
>  drivers/net/xen-netback/netback.c |   17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> index 43341b8..0c8ca76 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1351,14 +1351,15 @@ static bool tx_credit_exceeded(struct xenvif *vif,
> unsigned size)
>  	return false;
>  }
> 
> -static unsigned xenvif_tx_build_gops(struct xenvif *vif)
> +static unsigned xenvif_tx_build_gops(struct xenvif *vif, int budget)
>  {
>  	struct gnttab_copy *gop = vif->tx_copy_ops, *request_gop;
>  	struct sk_buff *skb;
>  	int ret;
> 
>  	while ((nr_pending_reqs(vif) + XEN_NETBK_LEGACY_SLOTS_MAX
> -		< MAX_PENDING_REQS)) {
> +		< MAX_PENDING_REQS) &&
> +	       (skb_queue_len(&vif->tx_queue) < budget)) {
>  		struct xen_netif_tx_request txreq;
>  		struct xen_netif_tx_request
> txfrags[XEN_NETBK_LEGACY_SLOTS_MAX];
>  		struct page *page;
> @@ -1380,8 +1381,7 @@ static unsigned xenvif_tx_build_gops(struct xenvif
> *vif)
>  			continue;
>  		}
> 
> -		RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx,
> work_to_do);
> -		if (!work_to_do)
> +		if (!RING_HAS_UNCONSUMED_REQUESTS(&vif->tx))
>  			break;
> 
>  		idx = vif->tx.req_cons;
> @@ -1520,14 +1520,13 @@ static unsigned xenvif_tx_build_gops(struct
> xenvif *vif)
>  }
> 
> 
> -static int xenvif_tx_submit(struct xenvif *vif, int budget)
> +static int xenvif_tx_submit(struct xenvif *vif)
>  {
>  	struct gnttab_copy *gop = vif->tx_copy_ops;
>  	struct sk_buff *skb;
>  	int work_done = 0;
> 
> -	while (work_done < budget &&
> -	       (skb = __skb_dequeue(&vif->tx_queue)) != NULL) {
> +	while ((skb = __skb_dequeue(&vif->tx_queue)) != NULL) {
>  		struct xen_netif_tx_request *txp;
>  		u16 pending_idx;
>  		unsigned data_len;
> @@ -1602,14 +1601,14 @@ int xenvif_tx_action(struct xenvif *vif, int
> budget)
>  	if (unlikely(!tx_work_todo(vif)))
>  		return 0;
> 
> -	nr_gops = xenvif_tx_build_gops(vif);
> +	nr_gops = xenvif_tx_build_gops(vif, budget);
> 
>  	if (nr_gops == 0)
>  		return 0;
> 
>  	gnttab_batch_copy(vif->tx_copy_ops, nr_gops);
> 
> -	work_done = xenvif_tx_submit(vif, nr_gops);
> +	work_done = xenvif_tx_submit(vif);
> 
>  	return work_done;
>  }
> --
> 1.7.10.4

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