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

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"))); \
> - 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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