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

[Jason Wang <jasowang@redhat.com>]

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<mst@redhat.com>
> > > > Cc: Qin Chuanyu<qinchuanyu@huawei.com>
> > > > Signed-off-by: Jason Wang<jasowang@redhat.com>
> > > 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.

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().

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.
> The key problem is how to release the memory of zero_copy's skb while
> been reserved.