Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751663Ab1EYEk6 (ORCPT ); Wed, 25 May 2011 00:40:58 -0400 Received: from ozlabs.org ([203.10.76.45]:45771 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750816Ab1EYEk4 (ORCPT ); Wed, 25 May 2011 00:40:56 -0400 From: Rusty Russell To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Carsten Otte , Christian Borntraeger , linux390@de.ibm.com, Martin Schwidefsky , Heiko Carstens , Shirley Ma , lguest@lists.ozlabs.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org, Krishna Kumar , Tom Lendacky , steved@us.ibm.com, habanero@linux.vnet.ibm.com Subject: Re: [PATCHv2 10/14] virtio_net: limit xmit polling In-Reply-To: <20110523111900.GB27212@redhat.com> References: <877h9kvlps.fsf@rustcorp.com.au> <20110522121008.GA12155@redhat.com> <87boyutbjg.fsf@rustcorp.com.au> <20110523111900.GB27212@redhat.com> User-Agent: Notmuch/0.5 (http://notmuchmail.org) Emacs/23.2.1 (i686-pc-linux-gnu) Date: Wed, 25 May 2011 10:58:26 +0930 Message-ID: <8762ozk1qd.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2045 Lines: 54 On Mon, 23 May 2011 14:19:00 +0300, "Michael S. Tsirkin" wrote: > On Mon, May 23, 2011 at 11:37:15AM +0930, Rusty Russell wrote: > > Can we hit problems with OOM? Sure, but no worse than now... > > The problem is that this "virtqueue_get_capacity()" returns the worst > > case, not the normal case. So using it is deceptive. > > > > Maybe just document this? Yes, but also by renaming virtqueue_get_capacity(). Takes it from a 3 to a 6 on the API hard-to-misuse scale. How about, virtqueue_min_capacity()? Makes the reader realize something weird is going on. > I still believe capacity really needs to be decided > at the virtqueue level, not in the driver. > E.g. with indirect each skb uses a single entry: freeing > 1 small skb is always enough to have space for a large one. > > I do understand how it seems a waste to leave direct space > in the ring while we might in practice have space > due to indirect. Didn't come up with a nice way to > solve this yet - but 'no worse than now :)' Agreed. > > > I just wanted to localize the 2+MAX_SKB_FRAGS logic that tries to make > > > sure we have enough space in the buffer. Another way to do > > > that is with a define :). > > > > To do this properly, we should really be using the actual number of sg > > elements needed, but we'd have to do most of xmit_skb beforehand so we > > know how many. > > > > Cheers, > > Rusty. > > Maybe I'm confused here. The problem isn't the failing > add_buf for the given skb IIUC. What we are trying to do here is stop > the queue *before xmit_skb fails*. We can't look at the > number of fragments in the current skb - the next one can be > much larger. That's why we check capacity after xmit_skb, > not before it, right? No, I was confused... More coffee! Thanks, Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/