Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 13 Feb 2020 14:07:20 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     John Ogness <john.ogness@linutronix.de>
Cc:     lijiang <lijiang@redhat.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] printk: replace ringbuffer
Message-ID: <20200213130720.j4e5qv37am2bapup@pathway.suse.cz>
References: <20200128161948.8524-1-john.ogness@linutronix.de>
 <dc4ca9b5-d2a2-03af-c186-204a3aad2399@redhat.com>
 <20200205044848.GH41358@google.com>
 <20200205050204.GI41358@google.com>
 <88827ae2-7af5-347b-29fb-cffb94350f8f@redhat.com>
 <20200205063640.GJ41358@google.com>
 <877e11h0ir.fsf@linutronix.de>
 <cd7509a5-48fd-e652-90f4-1e0fe2311134@redhat.com>
 <87sgjp9foj.fsf@linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87sgjp9foj.fsf@linutronix.de>
User-Agent: NeoMutt/20170912 (1.9.0)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed 2020-02-05 17:12:12, John Ogness wrote:
> On 2020-02-05, lijiang <lijiang@redhat.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