From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH] net: orphan queued skbs if device tx can stall Date: Tue, 10 Apr 2012 14:25:00 +0300 Message-ID: <20120410112459.GA28825@redhat.com> References: <20120408171323.GA16012@redhat.com> <1334044558.3126.5.camel@edumazet-glaptop> <20120410084151.GA27193@redhat.com> <1334048100.3126.21.camel@edumazet-glaptop> <20120410093140.GA27651@redhat.com> <1334052259.3126.68.camel@edumazet-glaptop> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "David S. Miller" , Jamal Hadi Salim , Stephen Hemminger , Jason Wang , Neil Horman , Jiri Pirko , Jeff Kirsher , =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= , Ben Hutchings , Herbert Xu To: Eric Dumazet Return-path: Content-Disposition: inline In-Reply-To: <1334052259.3126.68.camel@edumazet-glaptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, Apr 10, 2012 at 12:04:19PM +0200, Eric Dumazet wrote: > On Tue, 2012-04-10 at 12:31 +0300, Michael S. Tsirkin wrote: > > > True. Still this is the only interface we have for controlling > > the internal queue length so it seems safe to assume someone > > is using it for this purpose. > > > > So to workaround a problem in tun, you want to hack net/core/dev.c :( Sorry about being unclear, I'm just saying that your patch assumes tx_queue_len == 0 since you set it that way at device init but we can't rely on this as existing users might have changed that value. One way to fix would be a patch at the bottom: then we can leave tun to treat tx_queue_len like it always did. > Packets in qdisc should not be orphaned. > > If you think about it, why do we attach skb to socket in the first > place ? Good point. The answer is to avoid skb drops for local sockets by stopping them, right? > If its not needed for tun, why should it be needed for other devices ? Maybe needed but it's already broken for tun since the skbs in the private queue are orphaned by skb_orphan_try? > If TUN has a problem being stopped forever, maybe it should take > appropriate action to flush all packets in qdisc queue after a while, as > this makes no sense to delay packets forever. > Well arbitrary timers aren't a solid protection, right? We get out of stalling transmitters forever but a bad VM can still degrade performance significantly for others... ---- We don't want a queue for tun since it can stall forever, but userspace might tweak it's tx_queue_len as a way to control RX queue depth, and we don't want to break userspace. Use a private flag to disable queue. Warning: untested. Signed-off-by: Michael S. Tsirkin diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index 27883d1..644ca53 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -695,7 +692,7 @@ static void attach_one_default_qdisc(struct net_device *dev, { struct Qdisc *qdisc = &noqueue_qdisc; - if (dev->tx_queue_len) { + if (dev->tx_queue_len && !(dev->priv_flags & IFF_TX_CAN_STALL)) { qdisc = qdisc_create_dflt(dev_queue, &pfifo_fast_ops, TC_H_ROOT); if (!qdisc) { -- MST