linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Message-ID: <20170703153414.65ab12e3@gandalf.local.home>
Date:   Mon, 3 Jul 2017 15:34:14 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On Mon, 3 Jul 2017 20:11:30 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:

> +#define PRINTK_FLOOD_DEFAULT_DELAY	10
> +
>  int printk_delay_msec __read_mostly;
>  
> +static inline void __printk_delay(int m)
> +{
> +	while (m--) {
> +		mdelay(1);
> +		touch_nmi_watchdog();
> +	}
> +}
> +
>  static inline void printk_delay(void)
>  {
> -	if (unlikely(printk_delay_msec)) {
> -		int m = printk_delay_msec;
> +	unsigned long flags;
> +	u64 console_seen = 0, console_to_see;
>  
> -		while (m--) {
> -			mdelay(1);
> -			touch_nmi_watchdog();
> -		}
> +	if (printk_delay_msec) {
> +		__printk_delay(printk_delay_msec);
> +		return;
> +	}
> +

This had better be an option, and not default. And what happens if the
printk caller happens to preempt the one doing the writes to consoles?

-- Steve

> +	/*
> +	 * Check if consoles are far behind the loguf head and
> +	 * throttle printk() callers if so.
> +	 */
> +	logbuf_lock_irqsave(flags);
> +	if (console_seq > log_first_seq)
> +		console_seen = console_seq - log_first_seq;
> +	console_to_see = log_next_seq - console_seq;
> +	logbuf_unlock_irqrestore(flags);
> +
> +	if (console_seen < 4 * console_to_see) {
> +		if (printk_delay_msec)
> +			__printk_delay(printk_delay_msec);
> +		else
> +			__printk_delay(PRINTK_FLOOD_DEFAULT_DELAY);
>  	}
>  }
>