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

["Luis R. Rodriguez" <mcgrof@suse.com>]

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