linux-kernel - Re: [PATCH 1/7] printk: Hand over printing to console if printing too long

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Message-ID: <20151231024459.GA479@swordfish>
Date:	Thu, 31 Dec 2015 11:44:59 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	KY Sri nivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
 too long

Hello,

On (12/22/15 14:47), Jan Kara wrote:
[..]
> +int printk_deferred(const char *fmt, ...)
> +{
> +	va_list args;
> +	int r;
> +
> +	va_start(args, fmt);
> +	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
> +	va_end(args);
> +
> +	return r;
> +}
[..]
> @@ -1803,10 +1869,24 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	logbuf_cpu = UINT_MAX;
>  	raw_spin_unlock(&logbuf_lock);
>  	lockdep_on();
> +	/*
> +	 * By default we print message to console asynchronously so that kernel
> +	 * doesn't get stalled due to slow serial console. That can lead to
> +	 * softlockups, lost interrupts, or userspace timing out under heavy
> +	 * printing load.
> +	 *
> +	 * However we resort to synchronous printing of messages during early
> +	 * boot, when oops is in progress, or when synchronous printing was
> +	 * explicitely requested by kernel parameter.
> +	 */
> +	if (keventd_up() && !oops_in_progress && !sync_print) {
> +		__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> +		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> +	} else
> +		sync_print = true;
>  	local_irq_restore(flags);

So this fixes printk() and printk_deferred(), but it doesn't address any of the
direct and indirect console_lock/console_unlock callers.

for example, direct:
~/_mmots$ git grep console_unlock | egrep -v "printk\.c|panic\.c|console\.h" | wc -l
199

indirect (e.g. via console_devices()):
~/_mmots$ git grep console_device | egrep -v "printk\.c|panic\.c|console\.h|_console_device" | wc -l
4

One of those indirect callers is tty_lookup_driver(), called from tty_open(). Which
is quite big to ignore, I suspect.

A user space process opening a tty can end up doing that while (1) call_console_drivers()
loop, I suspect. At least nothing prevents it, at a glance.

A side note, isn't it too often to cond_resched() from console_unlock()? What if
we have 10000000 very short printk() messages (e.g. no more than 32 chars).

	-ss
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