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trace: always use the "nop" backend on events with the "disable" keyword

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Any event with the keyword/property "disable" generates an empty trace event
using the "nop" backend, regardless of the current backend.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
This commit is contained in:
Lluís 2011-08-31 20:31:38 +02:00 committed by Stefan Hajnoczi
parent 23d15e860b
commit dd215f646c
2 changed files with 18 additions and 24 deletions
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25
docs/tracing.txt
View File

@ -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

17
scripts/tracetool
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@ -506,21 +506,10 @@ convert()
# Skip comments and empty lines
test -z "${str%%#*}" && continue
# Process the line. The nop backend handles disabled lines.
disable="0"
if has_property "$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
# Process the line. The nop backend handles disabled lines.
if has_property "$str" "disable"; then
"lineto$1_nop" "$str"
else
"$process_line" "$str"
fi