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

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Date:   Thu, 07 Mar 2019 13:06:34 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.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 PATCH v1 08/25] printk: add ring buffer and kthread

On 2019-03-04, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> 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.

This was the intention. Although, as I mentioned in a previous
response[0], perhaps we should not loosen the requirements on write().

> 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().

I could see disabling preemption and interrupts for emergency messages
in the printk-kthread in order to synchronize against an irq_work
secondary printer as suggested in my response[0]. But I don't see an
advantage to disabling preemption in general for
call_console_drivers(). It is exactly that disable_preempt() that is so
harmful for realtime tasks.

John Ogness

[0] https://lkml.kernel.org/r/87lg1rggcz.fsf@linutronix.de