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). 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? > By adding the > information to the arguments, we can avoid the situation where the TSC > offset information is overwritten by other events. However, TSC offset > is not frequently changed and same information is output many times > because almost all data are waste. Therefore, only using tracepoint > is not good idea. > > So, I suggest a hybrid method; record TSC offset change events and read > the last TSC offset from procfs when collecting the trace data. > In particular, the method is as follows: > 1. Enable the tracepoint of TSC offset change and record the value > before and after changing > 2. Start tracing > 3. Stop tracing > 4. Collect trace data and read /proc/pid/kvm/* > 5. Check if any trace event recording the two TSC offsets exists > in the trace data > if(existing) => use trace event (flow 6) > else => use /proc/pid/kvm/* (flow 7) > 6. Apply two TSC offsets of the trace event to the trace data and > sort the trace data > (Ex.) > * => tracepoint of changing TSC offset > . => another trace event > > [START]............*............[END] > <----------> <----------> > previous current > TSC offset TSC offset > > 7. Apply TSC offset of /proc/pid/kvm/* to the trace data and > sort the trace data > (Ex.) > . => another trace event(not tracepoint of changing TSC offset) > > [START].........................[END] > <-----------------------> > current > TSC offset > > Thanks, > > -- > Yoshihiro YUNOMAE > Software Platform Research Dept. Linux Technology Center > Hitachi, Ltd., Yokohama Research Laboratory > E-mail: yoshihiro.yunomae.ez@xxxxxxxxxxx > -- 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
