Re: [PATCH] printk: Add best-effort printk() buffering.

[Joe Perches <joe@perches.com>]

On Sun, 2017-04-30 at 22:54 +0900, Tetsuo Handa wrote:
> Sometimes we want to printk() multiple lines in a group without being
> disturbed by concurrent printk() from interrupts and/or other threads.
> For example, mixed printk() output of multiple thread's dump makes it
> hard to interpret.
> 
> This patch introduces fixed-sized statically allocated buffers for
> buffering printk() output for each thread/context in best effort
> (i.e. up to PAGE_SIZE bytes, up to 16 concurrent printk() users).
[]
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
[]
> +#define MAX_PRINTK_BUFFERS 16
> +static struct printk_buffer {
> +	unsigned long context; /* printk_context() */
> +	unsigned int nested;
> +	unsigned int used; /* Valid bytes in buf[]. */
> +	char buf[PAGE_SIZE];
> +} printk_buffers[MAX_PRINTK_BUFFERS];

Perhaps these buffers could be acquired by
alloc_page rather than be static structures and
the sizeof buf[PAGE_SIZE] should be reduced by
	sizeof(unsigned long) +
	sizeof(unsigned int) +
	sizeof(unsigned int)
so that struct printk_buffers is exactly
PAGE_SIZE.