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authorPeter Maydell <peter.maydell@linaro.org>2020-01-20 15:11:40 +0000
committerStefan Hajnoczi <stefanha@redhat.com>2020-01-30 21:33:50 +0000
commit1aa6430dbee5237db8831f63b17b5c7c321098d0 (patch)
treee959893eb5261bc31fd85c97a48b9fd7a61c7d36 /docs
parent70149da7641815e08b985aa96ec15fe6ba927f12 (diff)
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>
Diffstat (limited to 'docs')
-rw-r--r--docs/devel/tracing.txt12
1 files changed, 5 insertions, 7 deletions
diff --git a/docs/devel/tracing.txt b/docs/devel/tracing.txt
index 8c0376fefa..cb5f685de9 100644
--- a/docs/devel/tracing.txt
+++ b/docs/devel/tracing.txt
@@ -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"