From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH] net: allow configuration of the size of page in __netdev_alloc_frag Date: Wed, 24 Oct 2012 14:28:23 +0200 Message-ID: <1351081703.6537.99.camel@edumazet-glaptop> References: <1351078936-14159-1-git-send-email-ian.campbell@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, Eric Dumazet , Konrad Rzeszutek Wilk , xen-devel@lists.xen.org To: Ian Campbell Return-path: Received: from mail-bk0-f46.google.com ([209.85.214.46]:64988 "EHLO mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757540Ab2JXM23 (ORCPT ); Wed, 24 Oct 2012 08:28:29 -0400 Received: by mail-bk0-f46.google.com with SMTP id jk13so215811bkc.19 for ; Wed, 24 Oct 2012 05:28:28 -0700 (PDT) In-Reply-To: <1351078936-14159-1-git-send-email-ian.campbell@citrix.com> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 2012-10-24 at 12:42 +0100, Ian Campbell wrote: > The commit 69b08f62e174 "net: use bigger pages in __netdev_alloc_frag" > lead to 70%+ packet loss under Xen when transmitting from physical (as > opposed to virtual) network devices. > > This is because under Xen pages which are contiguous in the physical > address space may not be contiguous in the DMA space, in fact it is > very likely that they are not. I think there are other architectures > where this is true, although perhaps non quite so aggressive as to > have this property at a per-order-0-page granularity. > > The real underlying bug here most likely lies in the swiotlb not > correctly handling compound pages, and Konrad is investigating this. > However even with the swiotlb issue fixed the current arrangement > seems likely to result in a lot of bounce buffering which seems likely > to more than offset any benefit from the use of larger pages. > > Therefore make NETDEV_FRAG_PAGE_MAX_ORDER configurable at runtime and > use this to request order-0 frags under Xen. Also expose this setting > via sysctl. > > Signed-off-by: Ian Campbell > Cc: Eric Dumazet > Cc: Konrad Rzeszutek Wilk > Cc: netdev@vger.kernel.org > Cc: xen-devel@lists.xen.org > --- I understand your concern, but this seems a quick/dirty hack at this moment. After setting the sysctl to 0, some tasks may still have some order-3 pages in their cache. Your driver must already cope with skb->head being split on several pages. So what fundamental difference exists with frags ?