From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: [PATCH net 2/3] xen-netback: worse-case estimate in xenvif_rx_action is underestimating Date: Thu, 27 Mar 2014 12:27:33 +0000 Message-ID: <1395923253.22909.106.camel@kazak.uk.xensource.com> References: <1395922988-37702-1-git-send-email-paul.durrant@citrix.com> <1395922988-37702-3-git-send-email-paul.durrant@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: , , Wei Liu , Sander Eikelenboom To: Paul Durrant Return-path: Received: from smtp.citrix.com ([66.165.176.89]:24276 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752124AbaC0M1f (ORCPT ); Thu, 27 Mar 2014 08:27:35 -0400 In-Reply-To: <1395922988-37702-3-git-send-email-paul.durrant@citrix.com> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, 2014-03-27 at 12:23 +0000, Paul Durrant wrote: > The worse-case estimate for skb ring slot usage in xenvif_rx_action() > fails to take fragment page_offset into account. The page_offset does, > however, affect the number of times the fragmentation code calls > start_new_rx_buffer() (i.e. consume another slot) and the worse-case > should assume that will always return true. This patch adds the page_offset > into the DIV_ROUND_UP for each frag. At least for the copying mode wasn't the idea that you would copy to the start of the page, so the offset wasn't relevant? IOW is the real issue that start_new_rx_buffer is/was too aggressive? Now that we do mapping though I suspect the offset becomes relevant again here and there is a 1:1 mapping from slots to frags again. (I could have sworn David V got rid of all this precalculating stuff.) > > Signed-off-by: Paul Durrant > Cc: Ian Campbell > Cc: Wei Liu > Cc: Sander Eikelenboom > --- > drivers/net/xen-netback/netback.c | 12 +++++++++++- > 1 file changed, 11 insertions(+), 1 deletion(-) > > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c > index befc413..ac35489 100644 > --- a/drivers/net/xen-netback/netback.c > +++ b/drivers/net/xen-netback/netback.c > @@ -492,8 +492,18 @@ static void xenvif_rx_action(struct xenvif *vif) > PAGE_SIZE); > for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) { > unsigned int size; > + unsigned int offset; > + > size = skb_frag_size(&skb_shinfo(skb)->frags[i]); > - max_slots_needed += DIV_ROUND_UP(size, PAGE_SIZE); > + offset = skb_shinfo(skb)->frags[i].page_offset; > + > + /* For a worse-case estimate we need to factor in > + * the fragment page offset as this will affect the > + * number of times xenvif_gop_frag_copy() will > + * call start_new_rx_buffer(). > + */ > + max_slots_needed += DIV_ROUND_UP(offset + size, > + PAGE_SIZE); > } > if (skb_is_gso(skb) && > (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||