linux-kernel - Re: Progress on system crash traces with LTTng using DAX and pmem

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Message-ID: <864133911.4806.1414681896478.JavaMail.zimbra@efficios.com>
Date:	Thu, 30 Oct 2014 15:11:36 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	lttng-dev <lttng-dev@...ts.lttng.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>
Subject: Re: Progress on system crash traces with LTTng using DAX and pmem

----- Original Message -----
> From: "Kirill A. Shutemov" <kirill@...temov.name>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: "Matthew Wilcox" <willy@...ux.intel.com>, "Ross Zwisler" <ross.zwisler@...ux.intel.com>, "lttng-dev"
> <lttng-dev@...ts.lttng.org>, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.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 ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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