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Message-ID: <20121116031946.GA23939@amt.cnet> Date: Fri, 16 Nov 2012 01:19:46 -0200 From: Marcelo Tosatti <mtosatti@...hat.com> To: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com> Cc: linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, Joerg Roedel <joerg.roedel@....com>, David Sharp <dhsharp@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>, Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>, yrl.pp-manager.tt@...achi.com, Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>, Thomas Gleixner <tglx@...utronix.de> Subject: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset On Wed, Nov 14, 2012 at 10:36:21AM +0900, Yoshihiro YUNOMAE wrote: > Hi All, > > The following patch set can make disordered trace data of a guest and a host > sorted in chronological order. > > In a virtualization environment, it is difficult to analyze performance > problems, such as a delay of I/O request on a guest. This is because multiple > guests operate on the host. One of approaches for solving such kind of problems > is to sort trace data of guests and the host in chronological order. > > After we applied the patch set(https://lkml.org/lkml/2012/11/13/588), raw TSC > can be chosen as a timestamp of ftrace. TSC is useful for merging trace data > in chronological order by two reasons. One of the reasons is that guests can > directly read raw TSC from the CPU using rdtsc operation. This means that raw > TSC value is not software clock like sched_clock, so we don't need to consider > about how the timestamp is calculated. The other is that TSC of recent x86 CPUs > is constantly incremented. This means that we don't need to worry about pace of > the timestamp. Therefore, choosing TSC as a timestamp for tracing is reasonable > to integrate trace data of guests and a host. > > Here, we need to consider about just one matter for using TSC on guests. TSC > value on a guest is always the host TSC plus the guest's "TSC offset". In other > words, to merge trace data using TSC as timestamp in chronological order, we > need to consider TSC offset of the guest. > > However, only the host kernel can read the TSC offset from VMCS and TSC offset > is not output in anywhere now. In other words, tools in userland cannot get > the TSC offset value, so we cannot merge trace data of guest and the host in > chronological order. Therefore, the TSC offset should be exported for userland > tools. > > In this patch set, TSC offset is exported by printk() on the host. I also > attached a tool for merging trace data of a guest and a host in chronological > order. > > <Example> > We assume that wakeup-latency for a command is big on a guest. Normally > we will use ftrace's wakeup-latency tracer or event tracer on the guest, but we > may not be able to solve this problem. This is because guests often exit to > the host for several reasons. In the next, we will use TSC as ftrace's timestamp > and record the trace data on the guest and the host. Then, we get following > data: > > /* guest data */ > comm-3826 [000] d...49836825726903: sched_wakeup: [detail] > comm-3826 [000] d...49836832225344: sched_switch: [detail] > /* host data */ > qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail] > qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail] > qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail] > qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail] > > Since TSC offset is not considered, these data cannot be merged. If this trace > data is shown like as follows, we will be able to understand the reason: > > qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail] > comm-3826 [000] d.h.49836825726903: sched_wakeup: [detail] <= > qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail] > qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail] > comm-3826 [000] d...49836832225344: sched_switch: [detail] <= > qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail] > qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail] > > In this case, we can understand wakeup-latency was big due to exit to host > twice. Getting this data sorted in chronological order is our goal. > > To merge the data like previous pattern, we apply this patch set. Then, we can > get TSC offset of the guest as follows: > > $ dmesg | grep kvm > [ 57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ## > ^^^^ ^^^^^^^^^^^^^^^^^^^^ | > PID TSC offset | > HOST TSC value --+ > > We use this TSC offset value to a merge script and obtain the following data: > > $ ./trace-merge.pl 18446743360465545001 host.data guest.data > h qemu-kvm-2687 [003] d...50550079203669: kvm_exit: [detail] > h qemu-kvm-2687 [003] d...50550079206816: kvm_entry: [detail] > g comm-3826 [000] d.h.50550079226331: sched_wakeup: [detail] <= > h qemu-kvm-2687 [003] d...50550079240656: kvm_exit: [detail] > h qemu-kvm-2687 [003] d...50550079243467: kvm_entry: [detail] > h qemu-kvm-2687 [003] d...50550079256103: kvm_exit: [detail] > h qemu-kvm-2687 [003] d...50550079268391: kvm_entry: [detail] > g comm-3826 [000] d...50550079279266: sched_switch: [detail] <= > h qemu-kvm-2687 [003] d...50550079280829: kvm_exit: [detail] > h qemu-kvm-2687 [003] d...50550079286028: kvm_entry: [detail] > | > \----guest/host > > In this summary, I suggest the patch which TSC offset for each guest can be > output on the host. The guest TSC can change (for example if TSC scaling is used). Moreover TSC offset can change, and you'd have to monitor that. What about a module option so that tsc_offset is written as zero (to be used as debugging tool). Then the following restrictions apply: - TSC must be synchronized across CPUs/VCPUS. - TSC must be reliable. Would that suffice? (a module option to kvm.ko, say zero_tsc_offset). > I chose printk() to output TSC offset value, but I think this is not the best > method. For example, defining as a tracepoint is one of the methods. In the > case, multiple buffers are needed to keep these data. > > I need your comments, thanks! > --- > > Yoshihiro YUNOMAE (2): > kvm/vmx: Print TSC_OFFSET information when TSC offset value is written to VMCS > tools: Add a tool for merging trace data of a guest and a host > > > arch/x86/kvm/vmx.c | 5 + > tools/scripts/trace-merge/trace-merge.pl | 109 ++++++++++++++++++++++++++++++ > 2 files changed, 114 insertions(+) > create mode 100755 tools/scripts/trace-merge/trace-merge.pl > > -- > Yoshihiro YUNOMAE > Software Platform Research Dept. Linux Technology Center > Hitachi, Ltd., Yokohama Research Laboratory > E-mail: yoshihiro.yunomae.ez@...achi.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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