diff options
| -rw-r--r-- | docs/tracing.txt | 25 | ||||
| -rwxr-xr-x | scripts/tracetool | 15 |
2 files changed, 17 insertions, 23 deletions
diff --git a/docs/tracing.txt b/docs/tracing.txt index 455da37969..85793cf173 100644 --- a/docs/tracing.txt +++ b/docs/tracing.txt @@ -12,15 +12,11 @@ for debugging, profiling, and observing execution. ./configure --trace-backend=simple make -2. Enable trace events you are interested in: - - $EDITOR trace-events # remove "disable" from events you want - -3. Run the virtual machine to produce a trace file: +2. Run the virtual machine to produce a trace file: qemu ... # your normal QEMU invocation -4. Pretty-print the binary trace file: +3. Pretty-print the binary trace file: ./simpletrace.py trace-events trace-* @@ -103,10 +99,11 @@ portability macros, ensure they are preceded and followed by double quotes: 4. Name trace events after their function. If there are multiple trace events in one function, append a unique distinguisher at the end of the name. -5. Declare trace events with the "disable" property. Some trace events can - produce a lot of output and users are typically only interested in a subset - of trace events. Marking trace events disabled by default saves the user - from having to manually disable noisy trace events. +5. If specific trace events are going to be called a huge number of times, this + might have a noticeable performance impact even when the trace events are + programmatically disabled. In this case you should declare the trace event + with the "disable" property, which will effectively disable it at compile + time (using the "nop" backend). == Generic interface and monitor commands == @@ -165,6 +162,9 @@ The "nop" backend generates empty trace event functions so that the compiler can optimize out trace events completely. This is the default and imposes no performance penalty. +Note that regardless of the selected trace backend, events with the "disable" +property will be generated with the "nop" backend. + === Stderr === The "stderr" backend sends trace events directly to standard error. This @@ -173,6 +173,11 @@ effectively turns trace events into debug printfs. This is the simplest backend and can be used together with existing code that uses DPRINTF(). +Note that with this backend trace events cannot be programmatically +enabled/disabled. Thus, in order to trim down the amount of output and the +performance impact of tracing, you might want to add the "disable" property in +the "trace-events" file for those events you are not interested in. + === Simpletrace === The "simple" backend supports common use cases and comes as part of the QEMU diff --git a/scripts/tracetool b/scripts/tracetool index e649a5b807..e2cf11784b 100755 --- a/scripts/tracetool +++ b/scripts/tracetool @@ -506,21 +506,10 @@ convert() # Skip comments and empty lines test -z "${str%%#*}" && continue + echo # Process the line. The nop backend handles disabled lines. - disable="0" if has_property "$str" "disable"; then - disable="1" - fi - echo - if [ "$disable" = "1" ]; then - # Pass the disabled state as an arg for the simple - # or DTrace backends which handle it dynamically. - # For all other backends, call lineto$1_nop() - if [ $backend = "simple" -o "$backend" = "dtrace" ]; then - "$process_line" "$str" - else - "lineto$1_nop" "${str##disable }" - fi + "lineto$1_nop" "$str" else "$process_line" "$str" fi |