Date: Sat, 13 Dec 2014 00:29:03 +0100
From: "Luis R. Rodriguez" <mcgrof@suse.com>
To: Vinson Lee <vlee@twopensource.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michal Hocko <mhocko@suse.cz>, Josh Triplett <josh@joshtriplett.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Fabian Frederick <fabf@skynet.be>,
        Bertrand Jacquin <beber@meleeweb.net>,
        Andy Lutomirski <luto@amacapital.net>,
        Pranith Kumar <bobby.prani@gmail.com>,
        Peter Foley <pefoley2@pefoley.com>,
        Clark Williams <clark.williams@gmail.com>,
        Mel Gorman <mgorman@suse.de>, LKML <linux-kernel@vger.kernel.org>,
        Nate Stahl <stahl@twitter.com>, Vinson Lee <vlee@twitter.com>
Subject: Re: [PATCH] Increase maximum size of dmesg buffer to 16MB.
Message-ID: <20141212232903.GV25677@wotan.suse.de>
References: <1418423906-5461-1-git-send-email-vlee@twopensource.com> <20141212224515.GU25677@wotan.suse.de> <CAHTgTXXo-DcmLH-JQ03B4zod_0qOhnJPU8Lc32U7A0PCZXFHuA@mail.gmail.com>
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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@suse.com> wrote:
> > On Fri, Dec 12, 2014 at 02:38:26PM -0800, Vinson Lee wrote:
> >> From: Nate Stahl <stahl@twitter.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@twitter.com>
> >> Signed-off-by: Vinson Lee <vlee@twitter.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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