On Wed, Jun 24, 2015 at 12:06:03PM -0500, Eric W. Biederman wrote: > dwalker@xxxxxxxxxx writes: > > > On Wed, Jun 24, 2015 at 03:52:48PM +0900, Masami Hiramatsu wrote: > >> Hi, > >> > >> On 2015/06/23 23:05, dwalker@xxxxxxxxxx wrote: > >> > > >> > Hi, > >> > > >> > There was a commit in kernel/panic.c which altered when the kexec crash kernel is executed, > >> > > >> > commit f06e5153f4ae2e2f3b0300f0e260e40cb7fefd45 > >> > Author: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> > >> > Date: Fri Jun 6 14:37:07 2014 -0700 > >> > > >> > kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers > >> > > >> > > >> > This made it possible for smp_send_stop() to be executed prior to calling the kexec crash > >> > kernel. > >> > > >> > The issue is that smp_send_stop() offlines the cores, and other code depend on the cores being online. > >> > > >> > In my case on Octeon here's an example, > >> > > >> > panic() > >> > crash_kexec() > >> > machine_crash_shutdown() > >> > octeon_generic_shutdown() > >> > > >> > Inside octeon_generic_shutdown() the Octeon watchdog is shutdown for_each_online_cpu(), but since > >> > most of the cpu's already got offlined in smp_send_stop() it means the watchdog is still alive on > >> > those cores. This results in a reboot during the crash kernel execution. > >> > >> Ah, I see. > >> > >> > Another example seem to be in default_machine_crash_shutdown() where crash_kexec_prepare_cpus() depends > >> > on an IPI for saving the registers on different cores. However, the cpu's are all offlined with > >> > interrupts disabled so they won't be running those IPI's in this case. > >> > > >> > I'm looking for any advice on how this should be fixed, or if it's already fixed. I'm not going to be > >> > submitting a patch so if anyone wants to submit one feel free to do so. > >> > >> Hmm, IMHO, when the cpu goes to offline in appropriate way(smp_send_stop), it should stop > >> watchdog timer on the offlined cpu too. > >> Or, you can also register crash handler which stops all watchdogs, but it's a bit tricky. > >> > > > > That doesn't really fix all the issue tho. As I was explaining generic MIPS code depends on the cpu's > > effectively being online for crash data collection (with an IPI). This issue may effect other architectures also, > > because smp_send_stop() offlines the cpu on other architectures also. I haven't surveyed the other architectures > > enough to know what issue could happen from this tho. > > > > Is it possible to move the smp_send_stop() below the notifiers ? I'm > > just throwing out ideas. > > In general if you want reliability don't pass the kernel command line to > run the panic notifiers. We use the panic notifiers to "fail over" , or switch to a working machine. We could do that in the crash kernel, but we would have to wait several seconds which is too long. Do you know of a more stable way to accomplish that ? Daniel
