Re: [tip:tracing/printk] tracing: optimize trace_printk()

[Steven Rostedt]

On Mon, 9 Mar 2009, Steven Rostedt wrote:

> 
> On Mon, 9 Mar 2009, Ingo Molnar wrote:
> > 
> > tracing: optimize trace_printk()
> > 
> > Impact: micro-optimization
> > 
> > trace_printk() does this unconditionally:
> > 
> > 	trace_printk_fmt = fmt;
> > 
> > Where trace_printk_fmt is an entry into a global array. This is
> > very SMP-unfriendly.
> > 
> > So only write it once per bootup.
> > 
> > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > LKML-Reference: <1236356510-8381-5-git-send-email-fweisbec@xxxxxxxxx>
> > Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
> > 
> > 
> > ---
> >  include/linux/kernel.h |   10 ++++++++--
> >  1 files changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 4e726b9..7742798 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -454,7 +454,10 @@ do {									\
> >  do {									\
> >  	static const char *trace_printk_fmt				\
> >  	__attribute__((section("__trace_printk_fmt")));			\

A I missed the semicolon. I wonder if we could make it a static init?

-- Steve

> > -	trace_printk_fmt = fmt;					\
> > +									\
> > +	if (!trace_printk_fmt)						\
> > +		trace_printk_fmt = fmt;					\
> > +									\
> 
> But this is a static init. That is, it is done at initialization and not
> every time.
> 
> The change actually slows down the system.
> 
> >  	__trace_printk_check_format(fmt, ##args);			\
> >  	__trace_printk(_THIS_IP_, trace_printk_fmt, ##args);		\
> >  } while (0)
> > @@ -467,7 +470,10 @@ __trace_printk(unsigned long ip, const char *fmt, ...)
> >  do {									\
> >  	static const char *trace_printk_fmt				\
> >  	__attribute__((section("__trace_printk_fmt")));			\
> > -	trace_printk_fmt = fmt;					\
> > +									\
> > +	if (!trace_printk_fmt)						\
> > +		trace_printk_fmt = fmt;					\
> > +									\
> 
> Same here.
> 
> -- Steve
> 
> >  	__ftrace_vprintk(_THIS_IP_, trace_printk_fmt, vargs);		\
> >  } while (0)
> >  
> > 
> 
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