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: <20140624002050.GC27687@wotan.suse.de>
Date:	Tue, 24 Jun 2014 02:20:50 +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 Mon, Jun 23, 2014 at 03:41:34PM -0700, Andrew Morton wrote:
> On Wed, 18 Jun 2014 13:45:37 -0700 "Luis R. Rodriguez" <mcgrof@...not-panic.com> wrote:
> 
> > ...
> >
> > If an increase is required the ring buffer is increased to
> > +	  the next power of 2 that can fit both the minimum kernel ring buffer
> > +	  (LOG_BUF_SHIFT) plus the additional worst case CPU contributions.
> >
> > ...
> >
> > +	log_buf_len_update(cpu_extra + __LOG_BUF_LEN);
> > +}
> 
> I'd have expected
> 
>   total_cpu_space = minimum-per-cpu-len * nr_possible_cpus;
>   log_buf_len = max(__LOG_BUF_LEN, total_cpu_space)
> 
> but here you added __LOG_BUF_LEN to total_cpu_space and I cannot work
> out why.  
> .

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.

  Luis
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