linux-kernel - Re: [PATCH] Increase maximum size of dmesg buffer to 16MB.

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Date:	Sat, 13 Dec 2014 00:29:03 +0100
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Vinson Lee <vlee@...pensource.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michal Hocko <mhocko@...e.cz>,
	Josh Triplett <josh@...htriplett.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Fabian Frederick <fabf@...net.be>,
	Bertrand Jacquin <beber@...eeweb.net>,
	Andy Lutomirski <luto@...capital.net>,
	Pranith Kumar <bobby.prani@...il.com>,
	Peter Foley <pefoley2@...oley.com>,
	Clark Williams <clark.williams@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Nate Stahl <stahl@...tter.com>, Vinson Lee <vlee@...tter.com>
Subject: Re: [PATCH] Increase maximum size of dmesg buffer to 16MB.

On Fri, Dec 12, 2014 at 03:07:23PM -0800, Vinson Lee wrote:
> On Fri, Dec 12, 2014 at 2:45 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> > On Fri, Dec 12, 2014 at 02:38:26PM -0800, Vinson Lee wrote:
> >> From: Nate Stahl <stahl@...tter.com>
> >>
> >> A full task stack dump of all tasks on a machine can generate more than
> >> 4MB of output to dmesg. Dumping this data to the serial console causes
> >> the machine to hang for a number of minutes (an unacceptable impact),
> >> but dumping the same data to memory is feasible if the dmesg buffer is
> >> sized large enough to hold the output. Set to 16MB which will hopefully
> >> be large enough to handle a dump from any of our servers at this time.
> >>
> >> Signed-off-by: Nate Stahl <stahl@...tter.com>
> >> Signed-off-by: Vinson Lee <vlee@...tter.com>
> >
> > Isn't this the perpetual issue of having large number of CPUs? If so
> > consider use of LOG_CPU_MAX_BUF_SHIFT instead, otherwise clarifying how
> > this would be a different issue would be good. LOG_CPU_MAX_BUF_SHIFT
> > should scale nicely but you can increase it as well, is it being used?
> >
> >  Luis
> 
> 
> No, it is not being used.
> 
> LOG_CPU_MAX_BUF_SHIFT is a 3.17+ config and we do not have any of the
> above mentioned production machines running 3.17 or later.

Then consider backporting it.

> This patch did help us with debugging when running on older stable kernels
> though.

Understood, give the other stuff a spin.

  Luis
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