netdev - Re: DQL and TCQ_F_CAN_BYPASS destroy performance under virtualizaiton (Was: "Re: net

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Date:   Tue, 9 May 2017 11:11:09 -0400
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Anton Ivanov <anton.ivanov@...bridgegreys.com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>, jasowang@...hat.com
Subject: Re: DQL and TCQ_F_CAN_BYPASS destroy performance under
 virtualizaiton (Was: "Re: net_sched strange in 4.11")

On Tue, May 09, 2017 at 08:46:46AM +0100, Anton Ivanov wrote:
> I have figured it out. Two issues.
> 
> 1) skb->xmit_more is hardly ever set under virtualization because the qdisc
> is usually bypassed because of TCQ_F_CAN_BYPASS. Once TCQ_F_CAN_BYPASS is
> set a virtual NIC driver is not likely see skb->xmit_more (this answers my
> "how does this work at all" question).
> 
> 2) If that flag is turned off (I patched sched_generic to turn it off in
> pfifo_fast while testing), DQL keeps xmit_more from being set. If the driver
> is not DQL enabled xmit_more is never ever set. If the driver is DQL enabled
> the queue is adjusted to ensure xmit_more stops happening within 10-15 xmit
> cycles.
> 
> That is plain *wrong* for virtual NICs - virtio, emulated NICs, etc. There,
> the BIG cost is telling the hypervisor that it needs to "kick" the packets.
> The cost of putting them into the vNIC buffers is negligible. You want
> xmit_more to happen - it makes between 50% and 300% (depending on vNIC
> design) difference. If there is no xmit_more the vNIC will immediately
> "kick" the hypervisor and try to signal that  the packet needs to move
> straight away (as for example in virtio_net).
> 
> In addition to that, the perceived line rate is proportional to this cost,
> so I am not sure that the current dql math holds. In fact, I think it does
> not - it is trying to adjust something which influences the perceived line
> rate.
> 
> So - how do we turn BOTH bypass and DQL adjustment while under
> virtualization and set them to be "always qdisc" + "always xmit_more
> allowed"
> 
> A.
> 
> P.S. Cc-ing virtio maintainer

CCing Michael Tsirkin and Jason Wang, who are the core virtio and
virtio-net maintainers.  (I maintain the vsock driver - it's unrelated
to this discussion.)

> 
> A.
> 
> 
> On 08/05/17 08:15, Anton Ivanov wrote:
> > Hi all,
> > 
> > I was revising some of my old work for UML to prepare it for submission
> > and I noticed that skb->xmit_more does not seem to be set any more.
> > 
> > I traced the issue as far as net/sched/sched_generic.c
> > 
> > try_bulk_dequeue_skb() is never invoked (the drivers I am working on are
> > dql enabled so that is not the problem).
> > 
> > More interestingly, if I put a breakpoint and debug output into
> > dequeue_skb() around line 147 - right before the bulk: tag that skb
> > there is always NULL. ???
> > 
> > Similarly, debug in pfifo_fast_dequeue shows only NULLs being dequeued.
> > Again - ???
> > 
> > First and foremost, I apologize for the silly question, but how can this
> > work at all? I see the skbs showing up at the driver level, why are
> > NULLs being returned at qdisc dequeue and where do the skbs at the
> > driver level come from?
> > 
> > Second, where should I look to fix it?
> > 
> > A.
> > 
> 
> 
> -- 
> Anton R. Ivanov
> 
> Cambridge Greys Limited, England company No 10273661
> http://www.cambridgegreys.com/
> 

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