[lttng-dev] Progress on system crash traces with LTTng using DAX and pmem

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Thu Oct 30 11:11:36 EDT 2014

----- Original Message -----
> From: "Kirill A. Shutemov" <kirill at shutemov.name>
> To: "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> Cc: "Matthew Wilcox" <willy at linux.intel.com>, "Ross Zwisler" <ross.zwisler at linux.intel.com>, "lttng-dev"
> <lttng-dev at lists.lttng.org>, linux-fsdevel at vger.kernel.org, linux-mm at kvack.org, linux-kernel at vger.kernel.org
> Sent: Tuesday, October 28, 2014 6:54:58 AM
> Subject: Re: Progress on system crash traces with LTTng using DAX and pmem
> On Sat, Oct 25, 2014 at 12:51:25PM +0000, Mathieu Desnoyers wrote:
> > FYI, the main reason why my customer wants to go with a
> > "trace into memory that survives soft reboot" approach
> > rather than to use things like kexec/kdump is that they
> > care about the amount of time it takes to reboot their
> > machines. They want a solution where they can extract the
> > detailed crash data after reboot, after the machine is
> > back online, rather than requiring a few minutes of offline
> > time to extract the crash details.
> IIRC, on x86 there's no guarantee that your memory content will be
> preserved over reboot. BIOS is free to mess with it.

Hi Kirill,

This is a good point,

There are a few more aspects to consider here:

- Other architectures appear to have different guarantees, for
  instance ARM which, AFAIK, does not reset memory on soft
  reboot (well at least for my customer's boards). So I guess
  if x86 wants to be competitive, it would be good for them to
  offer a similar feature,

- Already having a subset of machines supporting this is useful,
  e.g. storing trace buffers and recovering them after a crash,

- Since we are in a world of dynamically upgradable BIOS, perhaps
  if we can show that there is value in having a BIOS option to
  specify a memory range that should not be reset on soft reboot,
  BIOS vendors might be inclined to include an option for it,

- Perhaps UEFI BIOS already have some way of specifying that a
  memory range should not be reset on soft reboot ?

Thoughts ?



Mathieu Desnoyers
EfficiOS Inc.

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