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

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Message-ID: <20200214010755.GA13877@google.com>
Date:   Fri, 14 Feb 2020 10:07:55 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        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 (20/02/13 14:07), Petr Mladek wrote:
> 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.

[..]

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

Maybe we can have CONFIG_PRINTK_EXTRA_PAYLOAD [for dicts] so people can
compile it out if it's not needed. This can save several bytes here and
there.

	-ss