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<mst@xxxxxxxxxx> > >>>>>> > >>>>Cc: Qin Chuanyu<qinchuanyu@xxxxxxxxxx> > >>>>>> > >>>>Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx> > >>>>> > >>>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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
