Trace Agent for virtio-trace
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============================
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Trace agent is a user tool for sending trace data of a guest to a Host in low
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overhead. Trace agent has the following functions:
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- splice a page of ring-buffer to read_pipe without memory copying
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- splice the page from write_pipe to virtio-console without memory copying
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- write trace data to stdout by using -o option
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- controlled by start/stop orders from a Host
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The trace agent operates as follows:
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1) Initialize all structures.
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2) Create a read/write thread per CPU. Each thread is bound to a CPU.
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The read/write threads hold it.
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3) A controller thread does poll() for a start order of a host.
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4) After the controller of the trace agent receives a start order from a host,
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the controller wake read/write threads.
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5) The read/write threads start to read trace data from ring-buffers and
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write the data to virtio-serial.
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6) If the controller receives a stop order from a host, the read/write threads
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stop to read trace data.
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Files
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=====
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README: this file
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Makefile: Makefile of trace agent for virtio-trace
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trace-agent.c: includes main function, sets up for operating trace agent
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trace-agent.h: includes all structures and some macros
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trace-agent-ctl.c: includes controller function for read/write threads
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trace-agent-rw.c: includes read/write threads function
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Setup
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=====
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To use this trace agent for virtio-trace, we need to prepare some virtio-serial
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I/Fs.
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1) Make FIFO in a host
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virtio-trace uses virtio-serial pipe as trace data paths as to the number
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of CPUs and a control path, so FIFO (named pipe) should be created as follows:
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# mkdir /tmp/virtio-trace/
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# mkfifo /tmp/virtio-trace/trace-path-cpu{0,1,2,...,X}.{in,out}
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# mkfifo /tmp/virtio-trace/agent-ctl-path.{in,out}
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For example, if a guest use three CPUs, the names are
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trace-path-cpu{0,1,2}.{in.out}
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and
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agent-ctl-path.{in,out}.
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2) Set up of virtio-serial pipe in a host
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Add qemu option to use virtio-serial pipe.
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##virtio-serial device##
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-device virtio-serial-pci,id=virtio-serial0\
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##control path##
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-chardev pipe,id=charchannel0,path=/tmp/virtio-trace/agent-ctl-path\
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-device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,\
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id=channel0,name=agent-ctl-path\
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##data path##
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-chardev pipe,id=charchannel1,path=/tmp/virtio-trace/trace-path-cpu0\
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-device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel0,\
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id=channel1,name=trace-path-cpu0\
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...
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If you manage guests with libvirt, add the following tags to domain XML files.
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Then, libvirt passes the same command option to qemu.
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<channel type='pipe'>
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<source path='/tmp/virtio-trace/agent-ctl-path'/>
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<target type='virtio' name='agent-ctl-path'/>
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<address type='virtio-serial' controller='0' bus='0' port='0'/>
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</channel>
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<channel type='pipe'>
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<source path='/tmp/virtio-trace/trace-path-cpu0'/>
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<target type='virtio' name='trace-path-cpu0'/>
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<address type='virtio-serial' controller='0' bus='0' port='1'/>
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</channel>
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...
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Here, chardev names are restricted to trace-path-cpuX and agent-ctl-path. For
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example, if a guest use three CPUs, chardev names should be trace-path-cpu0,
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trace-path-cpu1, trace-path-cpu2, and agent-ctl-path.
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3) Boot the guest
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You can find some chardev in /dev/virtio-ports/ in the guest.
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Run
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===
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0) Build trace agent in a guest
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$ make
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1) Enable ftrace in the guest
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<Example>
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# echo 1 > /sys/kernel/debug/tracing/events/sched/enable
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2) Run trace agent in the guest
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This agent must be operated as root.
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# ./trace-agent
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read/write threads in the agent wait for start order from host. If you add -o
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option, trace data are output via stdout in the guest.
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3) Open FIFO in a host
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# cat /tmp/virtio-trace/trace-path-cpu0.out
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If a host does not open these, trace data get stuck in buffers of virtio. Then,
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the guest will stop by specification of chardev in QEMU. This blocking mode may
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be solved in the future.
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4) Start to read trace data by ordering from a host
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A host injects read start order to the guest via virtio-serial.
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# echo 1 > /tmp/virtio-trace/agent-ctl-path.in
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5) Stop to read trace data by ordering from a host
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A host injects read stop order to the guest via virtio-serial.
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# echo 0 > /tmp/virtio-trace/agent-ctl-path.in
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