linux-kernel - Re: [PATCH 0/2] printk: replace ringbuffer

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Date:   Thu, 13 Feb 2020 14:07:20 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     lijiang <lijiang@...hat.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] printk: replace ringbuffer

On Wed 2020-02-05 17:12:12, John Ogness wrote:
> On 2020-02-05, lijiang <lijiang@...hat.com> wrote:
> > Do you have any suggestions about the size of CONFIG_LOG_* and
> > CONFIG_PRINTK_* options by default?
> 
> The new printk implementation consumes more than double the memory that
> the current printk implementation requires. This is because dictionaries
> and meta-data are now stored separately.
> 
> If the old defaults (LOG_BUF_SHIFT=17 LOG_CPU_MAX_BUF_SHIFT=12) were
> chosen because they are maximally acceptable defaults, then the defaults
> should be reduced by 1 so that the final size is "similar" to the
> current implementation.
>
> If instead the defaults are left as-is, a machine with less than 64 CPUs
> will reserve 336KiB for printk information (128KiB text, 128KiB
> dictionary, 80KiB meta-data).
> 
> It might also be desirable to reduce the dictionary size (maybe 1/4 the
> size of text?).

Good questions. It would be great to check the usage on some real
systems.

In each case, we should inform users when messages and/or dictionaries
were lost.

Also it would be great to have a way (function) that would show how
big parts of the two ring buffers are occupied by valid data. It might
be useful also to detect problems with the ring buffer:

   + too many space reserved but not commited

   + too many records invalidated because of different ordering
     in desc ring and data ring.


> However, since the new printk implementation allows for
> non-intrusive dictionaries, we might see their usage increase and start
> to be as large as the messages themselves.

I wish the dictionaries were never added ;-) They complicate the code
and nobody knows how many people actually use the information.

Best Regards,
Petr