linux-kernel - Re: [RFC][PATCHv2 2/4] printk: move printk

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Date:   Tue, 16 Oct 2018 16:21:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk
 header

On Tue, Oct 16, 2018 at 02:54:15PM +0200, Peter Zijlstra wrote:
> printk will determine the current context:
> 
>   task, softirq, hardirq or NMI
> 
> and pick the corresponding per-cpu line buffer and do the vsnprintf()

We need 4, but we don't need to do the exact context determination for
this. We can keep a simple counter:

#define MAX_IDX 4	/* task, sirq, hirq, nmi */
#define MAX_LEN 1020	/* sizeof(struct line_buffer) < 4k */

struct line_buffers {
	int idx;
	char line[MAX_IDX][MAX_LEN];
};

static DEFINE_PER_CPU(struct line_buffers, lbs);

static char *get_linebuf(void)
{
	struct line_buffers *lbp = this_cpu_ptr(&lbs);
	int idx;

	idx = lbp->idx++;
	return lbp->linx[idx];
}

static void put_linbuf(void)
{
	this_cpu_dec(lbs.idx);
}

> thing. Then we have the actual line length and content. With the length
> we reserve the bytes from the global buffer, we memcpy into the buffer
> and commit.