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

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Date:	Fri, 27 Jun 2014 01:16:30 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	gregkh@...uxfoundation.org, hpa@...ux.intel.com,
	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, Jun 26, 2014 at 02:41:17PM -0700, Andrew Morton wrote:
> On Tue, 24 Jun 2014 03:05:54 +0200 "Luis R. Rodriguez" <mcgrof@...e.com> wrote:
> 
> > > > Ah, because its cpu_extra, not total_cpu_space that is being
> > > > computed, the goal was to see how much extra junk on the
> > > > worst case a CPU might contribute. The __LOG_BUF_LEN is the
> > > > default size, so we combine both.
> > > 
> > > Well...  why?  Isn't it simpler and more direct to say "I want at least
> > > 32k per CPU"?
> > 
> > That's certainly another way to go about this, but the original motivation
> > was trying to figure out the additional *extra* junk a CPU might spewed out,
> > it set out with an assumption of a base start from the first CPU booting the
> > system and that first CPU using the default kernel ring buffer size. The
> > language in the patch describes the worst case extra CPU junk contributed,
> > rather than a specific full split of the kernel ring buffer as that's typically
> > how extra junk is spewered out to the ring bufer and the concern. In general
> > on idle each CPU only contributes about only 1 to max 2 lines. The focus then
> > is the worst case on contribution.
> 
> I don't think I understood all that ;)

Yeah if that made *you* squint a simpler approach would be better, regardless
of how technically correct the above explanation may be.

> > 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?

  Luis
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