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

[Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>]

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@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?

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.

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