Re: Re: [PATCH V2 1/1] kvm/vmx: Add a tracepoint write_tsc_offset

[Gleb Natapov]

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@xxxxxxxxxxx>
> >>>Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> >>>Cc: Gleb Natapov <gleb@xxxxxxxxxx>
> >>>Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >>>Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >>>Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> >>>---
> >>>  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@xxxxxxxxxxx
> 

--
			Gleb.
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