Messages in this thread | | | Date | Sun, 9 Jun 2013 14:14:42 +0300 | From | Gleb Natapov <> | Subject | Re: Re: [PATCH V2 1/1] kvm/vmx: Add a tracepoint write_tsc_offset |
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On Fri, Jun 07, 2013 at 02:22:22PM +0900, Yoshihiro YUNOMAE wrote: > (2013/06/06 20:33), Gleb Natapov wrote: > >On Wed, Jun 05, 2013 at 09:23:22PM -0300, Marcelo Tosatti wrote: > >>On Tue, Jun 04, 2013 at 05:36:19PM +0900, Yoshihiro YUNOMAE wrote: > >>>Add a tracepoint write_tsc_offset for tracing TSC offset change. > >>>We want to merge ftrace's trace data of guest OSs and the host OS using > >>>TSC for timestamp in chronological order. We need "TSC offset" values for > >>>each guest when merge those because the TSC value on a guest is always the > >>>host TSC plus guest's TSC offset. If we get the TSC offset values, we can > >>>calculate the host TSC value for each guest events from the TSC offset and > >>>the event TSC value. The host TSC values of the guest events are used when we > >>>want to merge trace data of guests and the host in chronological order. > >>>(Note: the trace_clock of both the host and the guest must be set x86-tsc in > >>>this case) > >>> > >>>TSC offset is stored in the VMCS by vmx_write_tsc_offset() or > >>>vmx_adjust_tsc_offset(). KVM executes the former function when a guest boots. > >>>The latter function is executed when kvm clock is updated. Only host can read > >>>TSC offset value from VMCS, so a host needs to output TSC offset value > >>>when TSC offset is changed. > >>> > >>>Since the TSC offset is not often changed, it could be overwritten by other > >>>frequent events while tracing. To avoid that, I recommend to use a special > >>>instance for getting this event: > >>> > >>>1. set a instance before booting a guest > >>> # cd /sys/kernel/debug/tracing/instances > >>> # mkdir tsc_offset > >>> # cd tsc_offset > >>> # echo x86-tsc > trace_clock > >>> # echo 1 > events/kvm/kvm_write_tsc_offset/enable > >>> > >>>2. boot a guest > >>> > >>>Signed-off-by: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com> > >>>Cc: Marcelo Tosatti <mtosatti@redhat.com> > >>>Cc: Gleb Natapov <gleb@redhat.com> > >>>Cc: Thomas Gleixner <tglx@linutronix.de> > >>>Cc: Ingo Molnar <mingo@redhat.com> > >>>Cc: "H. Peter Anvin" <hpa@zytor.com> > >>>--- > >>> arch/x86/kvm/trace.h | 18 ++++++++++++++++++ > >>> arch/x86/kvm/vmx.c | 3 +++ > >>> arch/x86/kvm/x86.c | 1 + > >>> 3 files changed, 22 insertions(+) > >>> > >>>diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h > >>>index fe5e00e..9c22e39 100644 > >>>--- a/arch/x86/kvm/trace.h > >>>+++ b/arch/x86/kvm/trace.h > >>>@@ -815,6 +815,24 @@ TRACE_EVENT(kvm_track_tsc, > >>> __print_symbolic(__entry->host_clock, host_clocks)) > >>> ); > >>> > >>>+TRACE_EVENT(kvm_write_tsc_offset, > >>>+ TP_PROTO(__u64 previous_tsc_offset, __u64 next_tsc_offset), > >>>+ TP_ARGS(previous_tsc_offset, next_tsc_offset), > >>>+ > >>>+ TP_STRUCT__entry( > >>>+ __field( __u64, previous_tsc_offset ) > >>>+ __field( __u64, next_tsc_offset ) > >>>+ ), > >>>+ > >>>+ TP_fast_assign( > >>>+ __entry->previous_tsc_offset = previous_tsc_offset; > >>>+ __entry->next_tsc_offset = next_tsc_offset; > >>>+ ), > >>>+ > >>>+ TP_printk("previous=%llu next=%llu", > >>>+ __entry->previous_tsc_offset, __entry->next_tsc_offset) > >>>+); > >>>+ > >> > >>Yoshihiro YUNOMAE, > >> > >>1) Why is previous_tsc_offset necessary? > > I was considering the situations where we did not enable > kvm_write_tsc_offset event before booting a guest or where we did not > use multiple buffers. Here, we will need another new I/F to get current > TSC offset of a given VCPU. For example, if kvm_write_tsc_offset is not > included in the host's trace data, we get the current TSC offset from > the new I/F and apply it to all guest events. On the other hand, if > kvm_write_tsc_offset event appears more than once, we apply the > previous offset to guest events before the first TSC offset change. > > Since we support only for using multiple buffers now, we don't need to > record previous TSC offset at this time. But I'm conscious that we have > to change the format of kvm_write_tsc_offset event when we support > those situations. > > >>2) The TSC offset traces should include vcpu number, so that its > >>possible to correlate traces of SMP guests (the tool should use > >>the individual vcpu tsc offsets when converting guests trace). > >> > >Why PID is not enough? No other trace, except kvm_entry, outputs vcpu id. > > As Gleb mentioned, a tool can understand TSC offset for each vcpu from > PID and vcpu number of kvm_entry. IMO, that is indirect way, so I would > be better off including vcpu number. > But doesn't the tool operates on vcpu's PID for all other events. I mean to figure out what vcpu an event belongs too during merge. Why tsc offset event is different?
> >>3) Please add traces for svm.c. > > Sure, I'll add the tracepoint for SVM. > > Thanks, > > Yoshihiro YUNOMAE > > -- > Yoshihiro YUNOMAE > Software Platform Research Dept. Linux Technology Center > Hitachi, Ltd., Yokohama Research Laboratory > E-mail: yoshihiro.yunomae.ez@hitachi.com >
-- Gleb.
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