Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset

[Yoshihiro YUNOMAE]

Hi Marcelo,

Sorry for the late reply.

(2012/11/17 4:15), Marcelo Tosatti wrote:
> On Wed, Nov 14, 2012 at 05:26:10PM +0900, Yoshihiro YUNOMAE wrote:
> > Thank you for commenting on my patch set.
> >
> > (2012/11/14 11:31), Steven Rostedt wrote:
> > > On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
> > > > On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > > > > On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
> > > > >
> > > > > > To merge the data like previous pattern, we apply this patch set. Then, we can
> > > > > > get TSC offset of the guest as follows:
> > > > > >
> > > > > > $ dmesg | grep kvm
> > > > > > [   57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
> > > > > >                       ^^^^                   ^^^^^^^^^^^^^^^^^^^^            |
> > > > > >                       PID                         TSC offset                 |
> > > > > >                                                             HOST TSC value --+
> > > > >
> > > > > Using printk to export something like this is IMO a nasty hack.
> > > > >
> > > > > Can't we create a /sys or /proc file to export the same thing?
> > > >
> > > > Since the value changes over the course of the trace, and seems to be
> > > > part of the context of the trace, I think I'd include it as a
> > > > tracepoint.
> > >
> > > I'm fine with that too.
> >
> > Using some tracepoint is a nice idea, but there is one problem. Here,
> > our discussion point is "the event which TSC offset is changed does not
> > frequently occur, but the buffer must keep the event data."
> >
> > There are two ideas for using tracepoint. First, we define new
> > tracepoint for changed TSC offset. This is simple and the overhead will
> > be low. However, this trace event stored in the buffer will be
> > overwritten by other trace events because this TSC offset event does
> > not frequently occur. Second, we add TSC offset information to the
> > tracepoint frequently occured. For example, we assume that TSC offset
> > information is added to arguments of trace_kvm_exit().
>
> The TSC offset is in the host trace. So given a host trace with two TSC
> offset updates, how do you know which events in the guest trace
> (containing a number of events) refer to which tsc offset update?
>
> Unless i am missing something, you can't solve this easily (well, except
> exporting information to the guest that allows it to transform RDTSC ->
> host TSC value, which can be done via pvclock).

As you say, TSC offset events are in the host trace, but we don't need
to notify guests of updating TSC offset. The offset event will output
the next TSC offset value and the current TSC value, so we can
calculate the guest TSC (T1) for the event. Guest TSCs since T1 can be
converted to host TSC using the TSC offset, so we can integrate those
trace data.

> Another issue as mentioned is lack of TSC synchronization in the host.
> Should you provide such a feature without the possibility of proper
> chronological order on systems with unsynchronized TSC?

I think, we cannot support this sorting feature using TSC on systems
with unsynchronized TSC. On systems with unsynchronized TSC, it is
difficult to sort not only trace data of guests and the host but trace
data of a guest or a host using TSC in chronological order. Actually,
if we want to output tracing data of ftrace in chronological order with
unsynchronized TSC, we will use the "global" mode as the timestamp. The
global mode uses wallclock added TSC correction, so the mode guarantees
to sort in chronological order for trace data of the guest or of
the host. If we use this mode to sort the trace data of guests and the
host in chronological order, we need to consider about the difference
between the guest and the host and timekeeping of guests and the host,
so it is difficult to solve these issues. At least, I haven't came up
with the good solution.

We cannot sort the trace data of guests and the host in chronological
order with unsynchronized TSC, but if we can set following
synchronization events for both guests and the host, we will know where
we should sort.

First, a guest and the host uses the global mode as the timestamp of
ftrace. Next, a user on the guest writes "1" to the synchronization I/F
as the ID, then the synchronization event "1" is recorded in a
ring-buffer of the guest. The synchronization operation induces
hypercall, so the host can handle the event. After the operation moves
to the host, the host records the event "1" in a ring-buffer of the
host. In the end, the operation returns to the host, and the
synchronization is finished.

When we integrate tracing data of the guest and the host, we
calculate difference of the timestamp between the synchronizing events
with the same ID. This value is a temporary "offset". We will convert
the timestamp of the guests to the timestamp of the host before the
next synchronizing event. If the synchronizing event cycle is very
short, we will not need to consider the timekeeping. Then, we can sort
the trace data in chronological order.

Would you comment for this or do you have another idea?

Thanks,
--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@xxxxxxxxxxx


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