Re: [PATCH 4/4] fsmonitor: Delay updating state until after split index is merged

> On Fri, Oct 20, 2017 at 03:16:20PM +0200, Johannes Schindelin wrote:
> 
> > >  void tweak_fsmonitor(struct index_state *istate)
> > >  {
> > > +	int i;
> > > +
> > > +	if (istate->fsmonitor_dirty) {
> > > +		/* Mark all entries valid */
> > > +		trace_printf_key(&trace_fsmonitor, "fsmonitor is enabled; cache is %d", istate->cache_nr);
> > 
> > Sadly, a call to trace_printf_key() is not really a noop when tracing is
> > disabled. The call to trace_printf_key() hands off to trace_vprintf_fl(),
> > which in turn calls prepare_trace_line() which asks trace_want() whether
> > we need to trace, which finally calls get_trace_fd(). This last function
> > initializes a trace key if needed, and this entire call stack takes time.
> 
> It seems like we could pretty easily turn noop traces into a trivial
> conditional, like:
> 
> diff --git a/trace.h b/trace.h
> index 179b249c59..c46b92cbde 100644
> --- a/trace.h
> +++ b/trace.h
> @@ -80,8 +80,11 @@ extern void trace_performance_since(uint64_t start, const char *format, ...);
>  #define trace_printf(...) \
>  	trace_printf_key_fl(TRACE_CONTEXT, __LINE__, NULL, __VA_ARGS__)
>  
> -#define trace_printf_key(key, ...) \
> -	trace_printf_key_fl(TRACE_CONTEXT, __LINE__, key, __VA_ARGS__)
> +#define trace_printf_key(key, ...) do { \
> +	if (!key->initialized || key->fd) \
> +		trace_printf_key_fl(TRACE_CONTEXT, __LINE__, key, __VA_ARGS__) \
> +} while(0)
> +
>  
>  #define trace_argv_printf(argv, ...) \
>  	trace_argv_printf_fl(TRACE_CONTEXT, __LINE__, argv, __VA_ARGS__)
> 
> 
> (OK, that's got an OR, but if we are really pinching instructions we
> could obviously store a single "I've been initialized and am disabled"
> flag).

> I don't have an opinion one way or the other on these particular
> messages, but in general I'd like to see _more_ tracing in Git, not
> less. I've often traced Git with a debugger or other tools like perf,
> but there's real value in the author of code annotating high-level
> logical events.