[Qemu-devel] [PATCH 0/2] New trace backend: ftrace

[Eiichi Tsukata]

This patchset introduces a new tracing backend "ftrace".

Currently, QEMU tracing backends do not support userspace tracing with ftrace.
Collecting QEMU trace data and kernel trace data simultaniouly is useful for
latency analysis and debugging especially when using KVM.

With ftrace backend, you can easily collect QEMU-kernel merged trace data
using existing ftrace event-based tracer. If you use KVM, you can effectively
compare VM_EXIT and QEMU userspace handler.

To try it out, compile QEMU with tracing backend ftrace, then enable KVM events
in ftrace:

    # echo 1 > /sys/kernel/debug/tracing/events/kvm/enable
    
After running qemu by root user, you can get the trace:
    
    # cat /sys/kernel/debug/tracing/trace

Example:
 # tracer: nop
 #
 # entries-in-buffer/entries-written: 8434/345512   #P:4
 #
 #                              _-----=> irqs-off
 #                             / _----=> need-resched
 #                            | / _---=> hardirq/softirq
 #                            || / _--=> preempt-depth
 #                            ||| /     delay
 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
 #              | |       |   ||||       |         |
 <snip>
 qemu-system-x86-31930 [000] d... 23580.595951: kvm_exit: reason IO_INSTRUCTION 
rip 0xc45d info 710048 0
 qemu-system-x86-31930 [000] .... 23580.595954: kvm_emulate_insn: f0000:c45d:e4 
71 (real)
 qemu-system-x86-31930 [000] .... 23580.595955: kvm_pio: pio_read at 0x71 size 
1 count 1
 qemu-system-x86-31930 [000] .... 23580.595956: kvm_userspace_exit: reason 
KVM_EXIT_IO (2)
 qemu-system-x86-31930 [000] ...1 23580.595959: tracing_mark_write: 
cpu_set_apic_base 00000000fee00900
 qemu-system-x86-31930 [000] ...1 23580.595961: tracing_mark_write: cpu_in addr 
0x71 value 0
 qemu-system-x86-31930 [000] d... 23580.595964: kvm_entry: vcpu 0
 <snip>

"tracing_mark_write: cpu_set_apic_base 00000000fee00900" and
"tracing_mark_write: cpu_in addr 0x71 value 0" are QEMU trace data.
Others are Kernel trace data.

Furthermore, the ftrace backend overhead is smaller than uprobe-based event
tracer or SystemTap. My microbenchmark shows that ftrace tracing backend
overhead is about 0.8us per tracepoint, whereas uprobe-based event tracer
or SystemTap overhead is about 2.0us.

Eiichi Tsukata (2):
  trace: Add ftrace tracing backend
  trace: document frace backend

 configure                           |   8 +++
 docs/tracing.txt                    |  16 ++++++
 scripts/tracetool/backend/ftrace.py |  53 +++++++++++++++++++
 trace/Makefile.objs                 |   1 +
 trace/ftrace.c                      | 102 ++++++++++++++++++++++++++++++++++++
 trace/ftrace.h                      |  10 ++++
 6 files changed, 190 insertions(+)
 create mode 100644 scripts/tracetool/backend/ftrace.py
 create mode 100644 trace/ftrace.c
 create mode 100644 trace/ftrace.h

-- 
1.8.1.4