Messages in this thread | | | Date | Mon, 10 Jun 2013 18:30:42 +0900 | From | Yoshihiro YUNOMAE <> | Subject | Re: Re: Re: [PATCH V2 1/1] kvm/vmx: Add a tracepoint write_tsc_offset |
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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
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