Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

[Steven Rostedt]

On Wed, 21 Jan 2009, Avi Kivity wrote:

> Kevin Shanahan wrote:
> > On Tue, 2009-01-20 at 19:47 +0200, Avi Kivity wrote:
> >   
> > > Steven Rostedt wrote:
> > >     
> > > > Note, the wakeup latency only tests realtime threads, since other
> > > > threads
> > > > can have other issues for wakeup. I could change the wakeup tracer as
> > > > wakeup_rt, and make a new "wakeup" that tests all threads, but it may
> > > > be difficult to get something accurate.
> > > >       
> > > Kevin, can you retest with kvm at realtime priority?
> > >     
> > 
> > Running vanilla Linux 2.6.28, kvm-82. First a control test to check that
> > the problem is still there when running at normal priority:
> > 
> > --- hermes-old.wumi.org.au ping statistics ---
> > 900 packets transmitted, 900 received, 0% packet loss, time 899283ms
> > rtt min/avg/max/mdev = 0.119/269.773/13739.426/1230.836 ms, pipe 14
> > 
> > Yeah, sure is.
> > 
> > Okay, so now I set the realtime attributes of the processes for the VM
> > instance being pinged:
> > 
> > flexo:~# ps ax | grep 6284
> >  6284 ?        Sl     6:11 /usr/local/kvm/bin/qemu-system-x86_64 -smp 2
> > -m 2048 -hda kvm-17-1.img -hdb kvm-17-tmp.img -net
> > nic,vlan=0,macaddr=52:54:00:12:34:67,model=rtl8139 -net
> > tap,vlan=0,ifname=tap17,script=no -vnc 127.0.0.1:17 -usbdevice tablet
> > -daemonize
> > flexo:~# pstree -p 6284
> > qemu-system-x86(6284)???{qemu-system-x86}(6285)
> >                       ??{qemu-system-x86}(6286)
> >                       ??{qemu-system-x86}(6540)
> > 
> > (info cpus on the QEMU console shows 6285 and 6286 being the VCPU
> > processes. Not sure what the third child is for, maybe vnc?.)
> > 
> > flexo:~# chrt -r -p 3 6284
> > flexo:~# chrt -r -p 3 6285
> > flexo:~# chrt -r -p 3 6286
> > flexo:~# chrt -p 6284
> > pid 6284's current scheduling policy: SCHED_RR
> > pid 6284's current scheduling priority: 3
> > flexo:~# chrt -p 6285
> > pid 6285's current scheduling policy: SCHED_RR
> > pid 6285's current scheduling priority: 3
> > flexo:~# chrt -p 6286
> > pid 6286's current scheduling policy: SCHED_RR
> > pid 6286's current scheduling priority: 3
> > 
> > And the result of the ping test now:
> > 
> > --- hermes-old.wumi.org.au ping statistics ---
> > 900 packets transmitted, 900 received, 0% packet loss, time 899326ms
> > rtt min/avg/max/mdev = 0.093/0.157/3.611/0.117 ms
> > 
> > So, a _huge_ difference. But what does it mean?
> 
> It means, a scheduling problem.  Can you run the latency tracer (which only
> works with realtime priority), so we can tell if it is (a) kvm failing to wake
> up the vcpu properly or (b) the scheduler delaying the vcpu from running.
> 

Note, I'm working on a tracer that will also measure non RT task wake up 
times.

-- Steve

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