linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Message-ID: <20190312123857.juatd6fwtfmqajze@pathway.suse.cz>
Date:   Tue, 12 Mar 2019 13:38:57 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        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 00/25] printk: new implementation

On Mon 2019-03-11 19:54:11, Sergey Senozhatsky wrote:
> On (03/07/19 10:53), John Ogness wrote:
> > Since all current console drivers are already irq safe, I'm
> > wondering if using irq_work to handle the emergency printing for console
> > drivers without write_atomic() would help. (If the printk caller is in a
> > context that write() supports, then write() could be called directly.)
> > This would also demand that the irq-safe requirements for write() are
> > not relaxed. The printk-kthread might still be faster than irq_work, but
> > it might increase reliability if an irq_work is triggered as an extra
> > precaution.
> 
> Hmm. OK. So one of the things with printk is that it's fully sequential.
> We call console drivers one by one. Slow consoles can affect what appears
> on the fast consoles; fast console have no impact on slow ones.
> 
> 	call_console_drivers()
> 		for_each_console(c)
> 			c->write(c, text, text_len);
> 
> So a list of (slow_serial serial netcon) console drivers is a camel train;
> fast netcon is not fast anymore, and slow consoles sometimes are the reason
> we have dropped messages. And if we drop messages we drop them for all
> consoles, including fast netcon. Turning that sequential pipline into a
> bunch of per-console kthreads/irq and letting fast consoles to be fast is
> not a completely bad thing. Let's think more about this, I'd like to read
> more opinions.

Per-console kthread sounds interesting but there is the problem with
reliability. I mean that kthread need not get scheduled.

Some of these problems might get solved by the per-console loglevel
patchset.

Sigh, any feature might be useful in some situation. But we always
have to consider the cost and the gain. I wonder how common is
to actively use two consoles at the same time and what would
be the motivation.

Best Regards,
Petr