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SubjectRe: [tip:tracing/printk] tracing: optimize trace_printk()
On Mon, Mar 09, 2009 at 01:24:50PM -0400, Steven Rostedt wrote:
>
>
>
> 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@gmail.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > LKML-Reference: <1236356510-8381-5-git-send-email-fweisbec@gmail.com>
> > > Signed-off-by: Ingo Molnar <mingo@elte.hu>
> > >
> > >
> > > ---
> > > 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


Actually I first tried to make it like this:
static const char *trace_printk_fmt
___attribute__((section("__trace_printk_fmt"))) = fmt;

But gcc slapped me. That's why I moved it in the following line.




> > > - 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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