linux-kernel - Re: [PATCH 1/3] add binary printf

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Message-ID: <20090226130243.GA22460@elte.hu>
Date:	Thu, 26 Feb 2009 14:02:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/3] add binary printf

* Frederic Weisbecker <fweisbec@...il.com> wrote:

> From: Lai Jiangshan <laijs@...fujitsu.com>
> 
> Impact: Add APIs for binary trace printk infrastructure
> 
> vbin_printf(): write args to binary buffer, string is copied
> when "%s" is occurred.
> 
> bstr_printf(): read from binary buffer for args and format a string
> 
> [fweisbec@...il.com: ported to latest -tip]
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  include/linux/string.h |    7 +
>  lib/Kconfig            |    3 +
>  lib/vsprintf.c         |  442 ++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 452 insertions(+), 0 deletions(-)

OK, it's a nice idea and speedup for printf based tracing - 
which is common and convenient. Would you mind to post the 
performance measurements you've done using the new bstr_printf() 
facility? (the nanoseconds latency figures you did in the timer 
irq in a system under load and on a system that is idle)

The new printf code itself should be done cleaner i think and is 
not acceptable in its current form.

These two new functions:

> +#ifdef CONFIG_BINARY_PRINTF
> +/*
> + * bprintf service:
> + * vbin_printf() - VA arguments to binary data
> + * bstr_printf() - Binary data to text string
> + */

Duplicate hundreds of lines of code into three large functions 
(vsnprintf, vbin_printf, bstr_printf). These functions only have 
a difference in the way the argument list is iterated and the 
way the parsed result is stored:

  vsnprintf:   iterates va_list, stores into string
  bstr_printf: iterates bin_buf, stores into string
  vbin_printf: iterates va_list, stores into bin_buf

We should try _much_ harder at unifying these functions before 
giving up and duplicating them...

An opaque in_buf/out_buf handle plus two helper function 
pointers passed in would be an obvious implementation.

That way we'd have a single generic (inline) function that knows 
about the printf format itself:

 __generic_printf(void *in_buf,
		  void *out_buf,
		  void * (*read_in_buf)(void **),
		  void * (*store_out_buf)(void **));

And we'd have various variants for read_in_buf and 
store_out_buf. The generic function iterates the following way:

	in_val = read_in_buf(&in_buf);
	...
	store_out_buf(&out_buf, in_val);

(where in_val is wide enough to store a single argument.) The 
iterators modify the in_buf / out_buf pointers. Argument 
skipping can be done by reading the in-buf and not using it. I 
think we can do it with just two iterator methods.

Or something like that - you get the idea. It can all be inlined 
so that we'd end up with essentially the same vsnprint() 
instruction sequence we have today.

	Ingo
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