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

From: Yoshihiro YUNOMAE
Date: Mon Jun 10 2013 - 05:30:51 EST

Hi Gleb,

(2013/06/09 20:14), Gleb Natapov wrote:
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))

+ 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?

In vcpu_load()@virt/kvm/kvm_main.c, it seems that PID of the vcpu thread
can be changed. Are you familiar with this situation?
If the situation can be occurred, outputting vcpu number is better, I
think. If not occurred, as you say, we will be able to merge those data
without vcpu number in write_tsc_offset event. However, when we
focus on output data of the write_tsc_offset event, it is difficult to
directly understand contents of the data if vcpu number information is
not included. So, including the information is useful, I think.

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

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