linux-kernel - Re: [PATCH] printk: Shrink printk_sched buffer size, eliminate it when !CONFIG

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Date:	Wed, 6 Jun 2012 09:33:22 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay@...y.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] printk: Shrink printk_sched buffer size, eliminate it
 when !CONFIG_PRINTK


* Joe Perches <joe@...ches.com> wrote:

> The size of the per-cpu printk_sched buf is much larger
> than necessary.  The maximum sched message emitted is
> ~80 bytes. Shrink the allocation for this printk_sched
> buffer from 512 bytes to 128.
> 
> printk_sched creates an unnecessary per-cpu buffer when
> CONFIG_PRINTK is not enabled.  Remove it when appropriate
> so embedded uses save a bit of space too.
> 
> Signed-off-by: Joe Perches <joe@...ches.com>
> ---
>  kernel/printk.c |   14 ++++++++++----
>  1 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/printk.c b/kernel/printk.c
> index 32462d2..61cff0b 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1726,24 +1726,30 @@ int is_console_locked(void)
>  }
>  
>  /*
> - * Delayed printk version, for scheduler-internal messages:
> + * Delayed printk version, for scheduler-internal messages.
> + * Not the normal 512 as it's a bit wasteful, sched messages are short,
> + * and 128 is more than sufficient for all current messages.
>   */
> -#define PRINTK_BUF_SIZE		512
> +#define PRINTK_SCHED_BUF_SIZE	128
>  
>  #define PRINTK_PENDING_WAKEUP	0x01
>  #define PRINTK_PENDING_SCHED	0x02
>  
>  static DEFINE_PER_CPU(int, printk_pending);
> -static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
> +#ifdef CONFIG_PRINTK
> +static DEFINE_PER_CPU(char [PRINTK_SCHED_BUF_SIZE], printk_sched_buf);
> +#endif
>  
>  void printk_tick(void)
>  {
>  	if (__this_cpu_read(printk_pending)) {
>  		int pending = __this_cpu_xchg(printk_pending, 0);
> +#ifdef CONFIG_PRINTK
>  		if (pending & PRINTK_PENDING_SCHED) {
>  			char *buf = __get_cpu_var(printk_sched_buf);
>  			printk(KERN_WARNING "[sched_delayed] %s", buf);
>  		}
> +#endif
>  		if (pending & PRINTK_PENDING_WAKEUP)
>  			wake_up_interruptible(&log_wait);
>  	}
> @@ -2189,7 +2195,7 @@ int printk_sched(const char *fmt, ...)
>  	buf = __get_cpu_var(printk_sched_buf);
>  
>  	va_start(args, fmt);
> -	r = vsnprintf(buf, PRINTK_BUF_SIZE, fmt, args);
> +	r = vsnprintf(buf, PRINTK_SCHED_BUF_SIZE, fmt, args);
>  	va_end(args);
>  
>  	__this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);

The change makes sense but the further proliferation of #ifdefs 
is rather ugly and shows confusion: fundamentally, if we are 
going to cut out more printk functionality in the !CONFIG_PRINTK 
we might as well disable the whole thing, not just the 
printk_sched bits.

Thanks,

	Ingo
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