On Thu 2017-04-27 10:31:18, Steven Rostedt wrote: > On Thu, 27 Apr 2017 15:38:19 +0200 > Petr Mladek <pmladek@xxxxxxxx> wrote: > > > > by the way, > > > does this `nmi_print_seq' bypass even fix anything for Steven? > > > > I think that this is the most important question. > > > > Steven, does the patch from > > https://lkml.kernel.org/r/20170420131154.GL3452@xxxxxxxxxxxxxxx > > help you to see the debug messages, please? > > You'll have to wait for a bit. The box that I was debugging takes 45 > minutes to reboot. And I don't have much more time to play on it before > I have to give it back. I already found the bug I was looking for and > I'm trying not to crash it again (due to the huge bring up time). I see. > When I get a chance, I'll see if I can insert a trigger to crash the > kernel from NMI on another box and see if this patch helps. I actually tested it here using this hack: diff --cc lib/nmi_backtrace.c index d531f85c0c9b,0bc0a3535a8a..000000000000 --- a/lib/nmi_backtrace.c +++ b/lib/nmi_backtrace.c @@@ -89,8 -90,7 +90,9 @@@ bool nmi_cpu_backtrace(struct pt_regs * int cpu = smp_processor_id(); if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) { + if (in_nmi()) + panic("Simulating panic in NMI\n"); + arch_spin_lock(&lock); if (regs && cpu_in_idle(instruction_pointer(regs))) { pr_warn("NMI backtrace for cpu %d skipped: idling at pc %#lx\n", cpu, instruction_pointer(regs)); and triggered by: echo l > /proc/sysrq-trigger The patch really helped to see much more (all) messages from the ftrace buffers in NMI mode. But the test is a bit artifical. The patch might not help when there is a big printk() activity on the system when the panic() is triggered. We might wrongly use the small per-CPU buffer when the logbuf_lock is tested and taken on another CPU at the same time. It means that it will not always help. I personally think that the patch might be good enough. I am not sure if a perfect (more comlpex) solution is worth it. Best Regards, Petr
