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

From: Jason Wang <[email-masked]>
Date: Thu, 27 Feb 2014 11:11:07 +0800

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?