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?
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