Commit 1aa6430d authored by Peter Maydell's avatar Peter Maydell Committed by Stefan Hajnoczi
Browse files

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: default avatarPeter 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: default avatarStefan Hajnoczi <stefanha@redhat.com>
parent 70149da7

docs/devel/tracing.txt

+5 −7
Original line number Diff line number Diff line
@@ -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"