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docs/devel/tracing.txt: Recommend only trace_event_get_state_backends() 

Instead of recommending checking the TRACE_FOO_ENABLED macro to
skip expensive computations needed only for tracing, recommend
only using trace_event_get_state_backends(). This works for both
compile-time and run-time disabling of events, and has no extra
performance impact if the event is compile-time disabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200120151142.18954-2-peter.maydell@linaro.org
Message-Id: <20200120151142.18954-2-peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
master
Peter Maydell 2020-01-20 15:11:40 +00:00 committed by Stefan Hajnoczi
parent 70149da764
commit 1aa6430dbe
1 changed files with 5 additions and 7 deletions
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12
docs/devel/tracing.txt
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@ -342,8 +342,10 @@ edit the "trace-events-all" 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:
function. In these cases you can use 'trace_event_get_state_backends()' to
guard such computations, so they are skipped if the event has been either
compile-time disabled or run-time disabled. If the event is compile-time
disabled, this check will have no performance impact.
#include "trace.h" /* needed for trace event prototype */
@ -356,7 +358,7 @@ guard such computations and avoid its compilation when the event is disabled:
align = getpagesize();
}
ptr = qemu_memalign(align, size);
if (TRACE_QEMU_VMALLOC_ENABLED) { /* preprocessor macro */
if (trace_event_get_state_backends(TRACE_QEMU_VMALLOC)) {
void *complex;
/* some complex computations to produce the 'complex' value */
trace_qemu_vmalloc(size, ptr, complex);
@ -364,10 +366,6 @@ guard such computations and avoid its compilation when the event is disabled:
return ptr;
}
You can check both if the event has been disabled and is dynamically enabled at
the same time using the 'trace_event_get_state_backends' routine (see header
"trace/control.h" for more information).
=== "tcg" ===
Guest code generated by TCG can be traced by defining an event with the "tcg"