linux-kernel - Re: [PATCH v8 4/4] printk: allow increasing the ring buffer depending on the number of CPUs

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Message-Id: <20140627165914.c41788af.akpm@linux-foundation.org>
Date:	Fri, 27 Jun 2014 16:59:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	hpa@...ux.intel.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Lunn <andrew@...n.ch>,
	Stephen Warren <swarren@...dotorg.org>,
	Michal Hocko <mhocko@...e.cz>, Petr Mladek <pmladek@...e.cz>,
	Joe Perches <joe@...ches.com>,
	Arun KS <arunks.linux@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [PATCH v8 4/4] printk: allow increasing the ring buffer
 depending on the number of CPUs

On Thu, 26 Jun 2014 16:32:15 -0700 "Luis R. Rodriguez" <mcgrof@...e.com> wrote:

> On Thu, Jun 26, 2014 at 4:20 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
> > On Fri, 27 Jun 2014 01:16:30 +0200 "Luis R. Rodriguez" <mcgrof@...e.com> wrote:
> >
> >> > > Another note --  since this option depends on SMP and !BASE_SMALL technically
> >> > > num_possible_cpus() won't ever return something smaller than or equal to 1
> >> > > but because of the default values chosen the -1 on the compuation does affect
> >> > > whether or not this will trigger on > 64 CPUs or >= 64 CPUs, keeping the
> >> > > -1 means we require > 64 CPUs.
> >> >
> >> > hm, that sounds like more complexity.
> >> >
> >> > > This all can be changed however we like but the language and explained logic
> >> > > would just need to be changed.
> >> >
> >> > Let's start out simple.  What's wrong with doing
> >> >
> >> >     log buf len = max(__LOG_BUF_LEN, nr_possible_cpus * per-cpu log buf len)
> >>
> >> Sure, you already took in the patch series though so how would you like to
> >> handle a respin, you just drop the last patch and we respin it?
> >
> > A fresh patch would suit.  That's if you think it is a reasonable
> > approach - you've thought about this stuff more than I have!
> 
> The way its implemented now makes more technical sense, in short it
> assumes the first boot (and CPU) gets the full default kernel ring
> buffer size, the extra size is for the gibberish that each extra CPU
> is expected to spew out in the worst case. What you propose makes the
> explanation simpler and easier to understand but sends the wrong
> message about exactly how the growth of the kernel ring buffer is
> expected scale with the addition of more CPUs.

OK, it's finally starting to sink in.  The model for the kernel-wide
printk output is "a great pile of CPU-independent stuff plus a certain
amount of per-cpu stuff".  And the code at present attempts to follow
that model.  Yes?

I'm rather internet-challenged at present - please let me take another look at
the patch on Monday.

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