Subject: Re: [PATCH net] vhost: net: switch to use data copy if pending DMAs exceed the limit

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 27 Feb 2014 12:17:05 +0200

On Thu, Feb 27, 2014 at 11:11:07AM +0800, Jason Wang wrote:
> On 02/26/2014 05:23 PM, Michael S. Tsirkin wrote:
> > On Wed, Feb 26, 2014 at 03:11:21PM +0800, Jason Wang wrote:
> >> > On 02/26/2014 02:32 PM, Qin Chuanyu wrote:
> >>> > >On 2014/2/26 13:53, Jason Wang wrote:
> >>>> > >>On 02/25/2014 09:57 PM, Michael S. Tsirkin wrote:
> >>>>> > >>>On Tue, Feb 25, 2014 at 02:53:58PM +0800, Jason Wang wrote:
> >>>>>> > >>>>We used to stop the handling of tx when the number of pending DMAs
> >>>>>> > >>>>exceeds VHOST_MAX_PEND. This is used to reduce the memory occupation
> >>>>>> > >>>>of both host and guest. But it was too aggressive in some cases, since
> >>>>>> > >>>>any delay or blocking of a single packet may delay or block the guest
> >>>>>> > >>>>transmission. Consider the following setup:
> >>>>>> > >>>>
> >>>>>> > >>>>     +-----+        +-----+
> >>>>>> > >>>>     | VM1 |        | VM2 |
> >>>>>> > >>>>     +--+--+        +--+--+
> >>>>>> > >>>>        |              |
> >>>>>> > >>>>     +--+--+        +--+--+
> >>>>>> > >>>>     | tap0|        | tap1|
> >>>>>> > >>>>     +--+--+        +--+--+
> >>>>>> > >>>>        |              |
> >>>>>> > >>>>     pfifo_fast   htb(10Mbit/s)
> >>>>>> > >>>>        |              |
> >>>>>> > >>>>     +--+--------------+---+
> >>>>>> > >>>>     |     bridge          |
> >>>>>> > >>>>     +--+------------------+
> >>>>>> > >>>>        |
> >>>>>> > >>>>     pfifo_fast
> >>>>>> > >>>>        |
> >>>>>> > >>>>     +-----+
> >>>>>> > >>>>     | eth0|(100Mbit/s)
> >>>>>> > >>>>     +-----+
> >>>>>> > >>>>
> >>>>>> > >>>>- start two VMs and connect them to a bridge
> >>>>>> > >>>>- add an physical card (100Mbit/s) to that bridge
> >>>>>> > >>>>- setup htb on tap1 and limit its throughput to 10Mbit/s
> >>>>>> > >>>>- run two netperfs in the same time, one is from VM1 to VM2.
> >>>>>> > >>>>Another is
> >>>>>> > >>>>   from VM1 to an external host through eth0.
> >>>>>> > >>>>- result shows that not only the VM1 to VM2 traffic were throttled but
> >>>>>> > >>>>   also the VM1 to external host through eth0 is also
> >>>>>> > >>>>throttled somehow.
> >>>>>> > >>>>
> >>>>>> > >>>>This is because the delay added by htb may lead the delay the finish
> >>>>>> > >>>>of DMAs and cause the pending DMAs for tap0 exceeds the limit
> >>>>>> > >>>>(VHOST_MAX_PEND). In this case vhost stop handling tx request until
> >>>>>> > >>>>htb send some packets. The problem here is all of the packets
> >>>>>> > >>>>transmission were blocked even if it does not go to VM2.
> >>>>>> > >>>>
> >>>>>> > >>>>We can solve this issue by relaxing it a little bit: switching to use
> >>>>>> > >>>>data copy instead of stopping tx when the number of pending DMAs
> >>>>>> > >>>>exceed the VHOST_MAX_PEND. This is safe because:
> >>>>>> > >>>>
> >>>>>> > >>>>- The number of pending DMAs were still limited by VHOST_MAX_PEND
> >>>>>> > >>>>- The out of order completion during mode switch can make sure that
> >>>>>> > >>>>   most of the tx buffers were freed in time in guest.
> >>>>>> > >>>>
> >>>>>> > >>>>So even if about 50% packets were delayed in zero-copy case, vhost
> >>>>>> > >>>>could continue to do the transmission through data copy in this case.
> >>>>>> > >>>>
> >>>>>> > >>>>Test result:
> >>>>>> > >>>>
> >>>>>> > >>>>Before this patch:
> >>>>>> > >>>>VM1 to VM2 throughput is 9.3Mbit/s
> >>>>>> > >>>>VM1 to External throughput is 40Mbit/s
> >>>>>> > >>>>
> >>>>>> > >>>>After this patch:
> >>>>>> > >>>>VM1 to VM2 throughput is 9.3Mbit/s
> >>>>>> > >>>>Vm1 to External throughput is 93Mbit/s
> >>>>> > >>>Would like to see CPU utilization #s as well.
> >>>>> > >>>
> >>>> > >>
> >>>> > >>Will measure this.
> >>>>>> > >>>>Simple performance test on 40gbe shows no obvious changes in
> >>>>>> > >>>>throughput after this patch.
> >>>>>> > >>>>
> >>>>>> > >>>>The patch only solve this issue when unlimited sndbuf. We still need a
> >>>>>> > >>>>solution for limited sndbuf.
> >>>>>> > >>>>
> >>>>>> > >>>>Cc: Michael S. Tsirkin<[email-masked]>
> >>>>>> > >>>>Cc: Qin Chuanyu<[email-masked]>
> >>>>>> > >>>>Signed-off-by: Jason Wang<[email-masked]>
> >>>>> > >>>I think this needs some thought.
> >>>>> > >>>
> >>>>> > >>>In particular I think this works because VHOST_MAX_PEND
> >>>>> > >>>is much smaller than the ring size.
> >>>>> > >>>Shouldn't max_pend then be tied to the ring size if it's small?
> >>>>> > >>>
> >>>> > >>
> >>>> > >>Yes it should. I just reuse the VHOST_MAX_PEND since it was there for a
> >>>> > >>long time.
> >>>>> > >>>Another question is about stopping vhost:
> >>>>> > >>>ATM it's waiting for skbs to complete.
> >>>>> > >>>Should we maybe hunt down skbs queued and destroy them
> >>>>> > >>>instead?
> >>>>> > >>>I think this happens when a device is removed.
> >>>>> > >>>
> >>>>> > >>>Thoughts?
> >>>>> > >>>
> >>>> > >>
> >>>> > >>Agree, vhost net removal should not be blocked by a skb. But since the
> >>>> > >>skbs could be queued may places, just destroy them may need extra locks.
> >>>> > >>
> >>>> > >>Haven't thought this deeply, but another possible sloution is to rcuify
> >>>> > >>destructor_arg and assign it to NULL during vhost_net removing.
> >>> > >
> >>> > >Xen treat it by a timer, for those skbs which has been delivered for a
> >>> > >while, netback would exchange page of zero_copy's skb with dom0's page.
> >>> > >
> >>> > >but there is still a race between host's another process handle the skb
> >>> > >and netback exchange its page. (This problem has been proved by testing)
> >>> > >
> >>> > >and Xen hasn't solved this problem yet, because if anyone want to solve
> >>> > >this problem completely, a page lock is necessary, but it would be
> >>> > >complex and expensive.
> >>> > >
> >>> > >rcuify destructor arg and assign it to NULL couldn't solve the problem
> >>> > >of page release that has been reserved by host's another process.
> >>> > >
> >> > 
> >> > There're two issues:
> >> > 
> >> > 1) if a zerocopy skb won't be freed or frags orphaned in time,
> >> > vhost_net removal will be blocked since it was waiting for the
> >> > refcnt of ubuf to zero.
> >> > 2) whether or not we should free all pending skbs during vhost_net removing.
> > all pending *zero copy* skbs.
> >
> >> > My proposal is for issue 1. Another idea is not wait for the refcnt
> >> > to be zero and then we can defer the freeing of vhost_net during the
> >> > release method of kref_put().
> > It's not freeing that's at issue. We must not access guest memory
> > after vhost stop, too.
> >
> >> > For issue 2, I'm still not sure we should do this or not. Looks like
> >> > there's a similar issue for the packets sent by tcp_sendpage() was
> >> > blocked or delayed.
> > What's the issue exactly? How would you trigger it?
> 
> I mean it looks similar to the issue that if we use vmsplice() to splice
> user pages to TCP socket, and then the packet were blocked or delayed by
> qdics or other. Did we wait for all pending packets in this case before
> terminating the process?

IIUC the socket object will hang around a while
but this won't block close().
-- 
MST