linux-kernel - Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

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Date:   Thu, 24 Sep 2020 15:17:46 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

On (20/09/22 17:44), John Ogness wrote:
> +/*
> + * The sprint buffers are used with interrupts disabled, so each CPU
> + * only requires 2 buffers: for non-NMI and NMI contexts. Recursive
> + * printk() calls are handled by the safe buffers.
> + */
> +#define SPRINT_CTX_DEPTH 2
> +
> +/* Static sprint buffers for early boot (only 1 CPU). */
> +static DECLARE_BITMAP(sprint_static_textbuf_map, SPRINT_CTX_DEPTH);
> +static char sprint_static_textbuf[SPRINT_CTX_DEPTH * LOG_LINE_MAX];
> +
> +/* Dynamically allocated sprint buffers. */
> +static unsigned int sprint_dynamic_textbuf_count;
> +static unsigned long *sprint_dynamic_textbuf_map;
> +static char *sprint_dynamic_textbuf;

Just a question:

Can dynamic_textbuf be a PER_CPU array of five textbuf[1024] buffers
(for normal printk, nmi, hard irq, soft irq and one extra buffer for
recursive printk calls)?

So then we'd

	vprintk(...)
	{
		preempt_disable();
		buf = this_cpu_ptr(... preempt_count_to_ctx());
		...
		preempt_enable();
	}

preempt_disable()/preempt_enable() is already in printk().

	-ss