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

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Mar 24, 2009 at 12:44:12PM +0100, Frederic Weisbecker wrote:
> On Sat, Mar 21, 2009 at 03:30:39PM +1030, Kevin Shanahan wrote:
> > On Thu, 2009-03-19 at 07:54 +1030, Kevin Shanahan wrote:
> > > On Wed, 2009-03-18 at 11:46 +1030, Kevin Shanahan wrote:
> > > > On Wed, 2009-03-18 at 01:20 +0100, Frederic Weisbecker wrote:
> > > > > Ok, I've made a small script based on yours which could do this job.
> > > > > You will just have to set yourself a threshold of latency
> > > > > that you consider as buggy. I don't remember the latency you observed.
> > > > > About 5 secs right?
> > > > > 
> > > > > It's the "thres" variable in the script.
> > > > > 
> > > > > The resulting trace should be a mixup of the function graph traces
> > > > > and scheduler events which look like this:
> > > > > 
> > > > >  gnome-screensav-4691  [000]  6716.774277:   4691:120:S ==> [000]     0:140:R <idle>
> > > > >   xfce4-terminal-4723  [001]  6716.774303:   4723:120:R   + [001]  4289:120:S Xorg
> > > > >   xfce4-terminal-4723  [001]  6716.774417:   4723:120:S ==> [001]  4289:120:R Xorg
> > > > >             Xorg-4289  [001]  6716.774427:   4289:120:S ==> [001]     0:140:R <idle>
> > > > > 
> > > > > + is a wakeup and ==> is a context switch.
> > > > > 
> > > > > The script will loop trying some pings and will only keep the trace that matches
> > > > > the latency threshold you defined.
> > > > > 
> > > > > Tell if the following script work for you.
> > > 
> > > ...
> > > 
> > > > Either way, I'll try to get some results in my maintenance window
> > > > tonight.
> > > 
> > > Testing did not go so well. I compiled and booted
> > > 2.6.29-rc8-tip-02630-g93c4989, but had some problems with the system
> > > load when I tried to start tracing - it shot up to around 16-20 or so. I
> > > started shutting down VMs to try and get it under control, but before I
> > > got back to tracing again the machine disappeared off the network -
> > > unresponsive to ping.
> > > 
> > > When I got in this morning, there was nothing on the console, nothing in
> > > the logs to show what went wrong. I will try again, but my next chance
> > > will probably be Saturday. Stay tuned.
> > 
> > Okay, new set of traces have been uploaded to:
> > 
> >   http://disenchant.net/tmp/bug-12465/trace-3/
> > 
> > These were done on the latest tip, which I pulled down this morning:
> > 2.6.29-rc8-tip-02744-gd9937cb.
> > 
> > The system load was very high again when I first tried to trace with
> > sevarl guests running, so I ended up only having the one guest running
> > and thankfully the bug was still reproducable that way.
> > 
> > Fingers crossed this set of traces is able to tell us something.
> > 
> > Regards,
> > Kevin.
> > 
> > 
> 
> Sorry, I've been late to answer.
> As I explained in my previous mail, you trace is only
> a snapshot that happened in 10 msec.
> 
> I experimented different sizes for the ring buffer but even
> a 1 second trace require 20 Mo of memory. And a so huge trace
> would be impractical.
> 
> I think we should keep the trace filters we had previously.
> If you don't minde, could you please retest against latest -tip
> the following updated patch? Iadded the filters, fixed the python
> subshell and also flushed the buffer more nicely according to
> a recent feature in -tip:
> 
> echo > trace 
> 
> instead of switching to nop.
> You will need to pull latest -tip again.
> 
> Thanks a lot Kevin!


Ah you will also need to increase the size of your buffer.
See below:
 
> 
> #!/bin/bash
> 
> # Switch off all CPUs except for one to simplify the trace
> echo 0 > /sys/devices/system/cpu/cpu1/online
> echo 0 > /sys/devices/system/cpu/cpu2/online
> echo 0 > /sys/devices/system/cpu/cpu3/online
> 
> 
> # Make sure debugfs has been mounted
> if [ ! -d /sys/kernel/debug/tracing ]; then
>     mount -t debugfs debugfs /sys/kernel/debug
> fi
> 
> # Set up the trace parameters
> pushd /sys/kernel/debug/tracing || exit 1
> echo 0 > tracing_enabled
> echo function_graph > current_tracer
> echo funcgraph-abstime > trace_options
> echo funcgraph-proc    > trace_options
> 
> # Set here the kvm IP addr
> addr="hermes-old"
> 
> # Set here a threshold of latency in sec
> thres="5000"
> found="False"
> lat=0
> prefix=/sys/kernel/debug/tracing
> 
> echo 1 > $prefix/events/sched/sched_wakeup/enable
> echo 1 > $prefix/events/sched/sched_switch/enable
> 
> # Set the filter for functions to trace
> echo ''         > set_ftrace_filter  # clear filter functions
> echo '*sched*' >> set_ftrace_filter 
> echo '*wake*'  >> set_ftrace_filter
> echo '*kvm*'   >> set_ftrace_filter
> 
> # Reset the function_graph tracer
> echo function_graph > $prefix/current_tracer

Put a

echo 20000 > $prefix/buffer_size_kb

So that we will have enough space (hopefully).

Thanks!

> 
> while [ "$found" != "True" ]
> do
>         # Flush the previous buffer
>         echo trace > $prefix/trace
> 
>         echo 1 > $prefix/tracing_enabled
>         lat=$(ping -c 1 $addr | grep rtt | grep -Eo " [0-9]+.[0-9]+")
>         echo 0 > $prefix/tracing_enabled
> 
> 	echo $lat
> 	found=$(python -c "print float(str($lat).strip())")
>         sleep 0.01
> done
> 
> echo 0 > $prefix/events/sched/sched_wakeup/enable
> echo 0 > $prefix/events/sched/sched_switch/enable
> 
> 
> echo "Found buggy latency: $lat"
> echo "Please send the trace you will find on $prefix/trace"
> 
>