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

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Date:   Tue, 12 Mar 2019 18:58:56 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        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
Subject: Re: [RFC PATCH v1 08/25] printk: add ring buffer and kthread

On (03/11/19 11:51), John Ogness wrote:
> > In new printk design the tasks are still affected by printing floods.
> > Tasks have to line up and (busy) wait for each other, regardless of
> > contexts.
> 
> They only line up and busy wait is to add the informational message to
> the ring buffer. The current printk implementation is the same in this
> respect. And as you've noted, the logbuf spinlock is not a source of
> latencies.

I was talking about prb_lock().

> > When I do ./a.out --loglevel=X  I have a clear understanding that
> > all messages which fall into [critical, X] range will be in the logs,
> > because I told that application that those messages are important to
> > me right now. And it used to be the same with the kernel loglevel.
> 
> The loglevel is not related to logging. It specifies the amount of
> console printing. But I will assume you are referring to creating log
> files by having an external device store the console printing.

Right. E.g. screenlog.0

> > But now the kernel will do its own thing:
> >
> >   - what the kernel considers important will go into the logs
> >   - what the kernel doesn't consider important _maybe_ will end up
> >     in the logs (preemptible printk kthread). And this is where
> >     loglevel now. After the _maybe_ part.
> 
> "what the kernel considers" is a configuration option of the
> administrator. The administrator can increase the verbocity of the
> console (loglevel) without having negative effects on the system
> itself. Also, if the system were to suddenly crash, those crash messages
> shouldn't be in jeopardy just because the verbocity of the console was
> turned up.

Right. I'm not very sure about yet another knob which everyone
should figure out. I guess I won't be surprised to find out that
people set it to loglevel value.

	-ss