haraldwolff/qemu-patch-raspberry4
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trace: Provide a per-event status define for conditional compilation

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Adds a 'TRACE_${NAME}_ENABLED' preprocessor define for each tracing event in
"trace.h".

This lets the user conditionally compile code with a relatively high execution
cost that is only necessary when producing the tracing information for an event
that is enabled.

Note that events using this define will probably have the "disable" property by
default, in order to avoid such costs on regular builds.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
This commit is contained in:
Lluís Vilanova 2011-12-06 17:38:15 +01:00 committed by Stefan Hajnoczi
parent a348f10884
commit b7d66a761f
2 changed files with 48 additions and 7 deletions
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46
docs/tracing.txt
View file

@ -98,12 +98,6 @@ respectively. This ensures portability between 32- and 64-bit platforms.
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. 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 ==
You can programmatically query and control the dynamic state of trace events
@ -234,3 +228,43 @@ probes:
--target-type system \
--target-arch x86_64 \
<trace-events >qemu.stp
== Trace event properties ==
Each event in the "trace-events" file can be prefixed with a space-separated
list of zero or more of the following event properties.
=== "disable" ===
If a specific trace event is going to be invoked a huge number of times, this
might have a noticeable performance impact even when the event is
programmatically disabled.
In this case you should declare such event with the "disable" property. This
will effectively disable the event at compile time (by using the "nop" backend),
thus having no performance impact at all on regular builds (i.e., unless you
edit the "trace-events" file).
In addition, there might be cases where relatively complex computations must be
performed to generate values that are only used as arguments for a trace
function. In these cases you can use the macro 'TRACE_${EVENT_NAME}_ENABLED' to
guard such computations and avoid its compilation when the event is disabled:
#include "trace.h" /* needed for trace event prototype */
void *qemu_vmalloc(size_t size)
{
void *ptr;
size_t align = QEMU_VMALLOC_ALIGN;
if (size < align) {
align = getpagesize();
}
ptr = qemu_memalign(align, size);
if (TRACE_QEMU_VMALLOC_ENABLED) { /* preprocessor macro */
void *complex;
/* some complex computations to produce the 'complex' value */
trace_qemu_vmalloc(size, ptr, complex);
}
return ptr;
}

9
scripts/tracetool
View file

@ -519,7 +519,7 @@ linetostap_end_dtrace()
# Process stdin by calling begin, line, and end functions for the backend
convert()
{
local begin process_line end str disable
local begin process_line end str name NAME enabled
begin="lineto$1_begin_$backend"
process_line="lineto$1_$backend"
end="lineto$1_end_$backend"
@ -534,8 +534,15 @@ convert()
# Process the line. The nop backend handles disabled lines.
if has_property "$str" "disable"; then
"lineto$1_nop" "$str"
enabled=0
else
"$process_line" "$str"
enabled=1
fi
if [ "$1" = "h" ]; then
name=$(get_name "$str")
NAME=$(echo $name | tr '[:lower:]' '[:upper:]')
echo "#define TRACE_${NAME}_ENABLED ${enabled}"
fi
done