diff options
| author | Peter Maydell <peter.maydell@linaro.org> | 2020-01-20 15:11:40 +0000 |
|---|---|---|
| committer | Stefan Hajnoczi <stefanha@redhat.com> | 2020-01-30 21:33:50 +0000 |
| commit | 1aa6430dbee5237db8831f63b17b5c7c321098d0 (patch) | |
| tree | e959893eb5261bc31fd85c97a48b9fd7a61c7d36 /docs | |
| parent | 70149da7641815e08b985aa96ec15fe6ba927f12 (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.txt | 12 |
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" |