linux-kernel - Re: [PATCH 0/5] Persist printk buffer across reboots.

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Message-ID: <4F5F0620.2020404@codeaurora.org>
Date:	Tue, 13 Mar 2012 01:32:32 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Avery Pennarun <apenwarr@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Fabio M. Di Nitto" <fdinitto@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Olaf Hering <olaf@...fle.de>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Tejun Heo <tj@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Yinghai LU <yinghai@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 0/5] Persist printk buffer across reboots.

On 3/12/2012 10:36 PM, Avery Pennarun wrote:
> The last patch in this series implements a new CONFIG_PRINTK_PERSIST option
> that, when enabled, puts the printk buffer in a well-defined memory location
> so that we can keep appending to it after a reboot.  The upshot is that,
> even after a kernel panic or non-panic hard lockup, on the next boot
> userspace will be able to grab the kernel messages leading up to it.  It
> could then upload the messages to a server (for example) to keep crash
> statistics.
>
> The preceding patches in the series are mostly just things I fixed up while
> working on that patch.
>
> Some notes:
>
> - I'm not totally sure of the locking or portability issues when calling
>    memblock or bootmem.  This all happens really early, and I *think*
>    interrupts are still disabled at that time, so it's probably okay.
>
> - Tested this version on x86 (kvm) and it works with soft reboot (ie. reboot
>    -f).  Since some BIOSes wipe the memory during boot, you might not have
>    any luck.  It should be great on many embedded systems, though, including
>    the MIPS system I've tested a variant of this patch on.  (Our MIPS build
>    is based on a slightly older kernel so it's not 100% the same, but I think
>    this should behave identically.)
>
> - The way we choose a well-defined memory location is slightly suspicious
>    (we just count down from the top of the address space) but I've tested it
>    pretty carefully, and it seems to be okay.
>
> - In printk.c with CONFIG_PRINTK_PERSIST set, we're #defining words like
>    log_end.  It might be cleaner to replace all instances of log_end with
>    LOG_END to make this more clear.  This is also the reason the struct
>    logbits members start with _: because otherwise they conflict with the
>    macro.  Suggestions welcome.

Android has something similar called ram_console (see 
staging/android/ram_console.c). The console is dumped to a ram buffer 
that is reserved very early in platform setup code. Then when the phone 
reboots you can cat /proc/last_kmsg to get the previous kernel message 
for debugging. Can you use that code?

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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