From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCHv2 10/14] virtio_net: limit xmit polling Date: Wed, 25 May 2011 10:58:26 +0930 Message-ID: <8762ozk1qd.fsf__554.225713904266$1306298575$gmane$org@rustcorp.com.au> References: <877h9kvlps.fsf@rustcorp.com.au> <20110522121008.GA12155@redhat.com> <87boyutbjg.fsf@rustcorp.com.au> <20110523111900.GB27212@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20110523111900.GB27212@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: Krishna Kumar , Carsten Otte , lguest@lists.ozlabs.org, Shirley Ma , kvm@vger.kernel.org, linux-s390@vger.kernel.org, netdev@vger.kernel.org, habanero@linux.vnet.ibm.com, Heiko Carstens , linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, steved@us.ibm.com, Christian Borntraeger , Tom Lendacky , Martin Schwidefsky , linux390@de.ibm.com List-Id: virtualization@lists.linuxfoundation.org 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.