linux-kernel - Re: [RFC PATCH v1 08/25] printk: add ring buffer and kthread

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Message-ID: <20190304073856.GA552@jagdpanzerIV>
Date:   Mon, 4 Mar 2019 16:38:56 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.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 PATCH v1 08/25] printk: add ring buffer and kthread

On (02/12/19 15:29), John Ogness wrote:
[..]
> +	/* the printk kthread never exits */
> +	for (;;) {
> +		ret = prb_iter_wait_next(&iter, buf,
> +					 PRINTK_RECORD_MAX, &master_seq);
> +		if (ret == -ERESTARTSYS) {
> +			continue;
> +		} else if (ret < 0) {
> +			/* iterator invalid, start over */
> +			prb_iter_init(&iter, &printk_rb, NULL);
> +			continue;
> +		}
> +
> +		msg = (struct printk_log *)buf;
> +		format_text(msg, master_seq, ext_text, &ext_len, text,
> +			    &len, printk_time);
> +
> +		console_lock();
> +		if (len > 0 || ext_len > 0) {
> +			call_console_drivers(ext_text, ext_len, text, len);
> +			boot_delay_msec(msg->level);
> +			printk_delay();
> +		}
> +		console_unlock();
> +	}

This, theoretically, creates a whole new world of possibilities for
console drivers. Now they can do GFP_KERNEL allocations and stall
printk_kthread during OOM; or they can explicitly reschedule from
->write() callback (via console_conditional_schedule()) because
console_lock() sets console_may_schedule.

It's one thing to do cond_resched() (or to let preemption to take over)
after call_console_drivers() (when we are done printing a message to all
console drivers) and another thing to let preemption to take over while
we are printing a messages to the consoles. It probably would make sense
to disable preemption around call_console_drivers().

	-ss