Re: [Qemu-devel] [PATCH 0/2] improve tracing

[Stefan Hajnoczi]

On Fri, Jul 21, 2017 at 08:04:17PM +0300, Lluís Vilanova wrote:
> Vladimir Sementsov-Ogievskiy writes:
> 
> > Current trace system have a drawback: parameters of trace functions
> > are calculated even if corresponding tracepoint is disabled. Also, it
> > looks like trace function are not actually inlined by compiler (at
> > least for me).
> 
> > Here is a fix proposal: move from function call to macros. Patch 02
> > is an example, of how to reduce extra calculations with help of
> > patch 01.
> 
> The tracing functions *were* inlined last time I checked, although things
> changed quite a lot since then. Not sure that will make a lot of difference in
> terms of overall performance (needs measuring).
> 
> As for arguments, each trace event has a define TRACE_{NAME}_ENABLED that you
> can use for that purpose. If this is not explained in tracing.txt, that is a
> documentation bug.

It is described in docs/devel/tracing.txt:

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;
    }

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