This allows eg. the RCU stall detector, or the soft/hardlockup
detectors to trigger a backtrace on all CPUs.

We implement this by sending a "safe" NMI, which will actually only
send an IPI. Unfortunately the generic code prints "NMI", so that's a
little confusing but we can probably live with it.

If one of the CPUs doesn't respond to the IPI, we then print some info
from it's paca and do a backtrace based on its saved_r1.

Example output:

  INFO: rcu_sched detected stalls on CPUs/tasks:
  	2-...0: (0 ticks this GP) idle=1be/1/4611686018427387904 softirq=1055/1055 fqs=25735
  	(detected by 4, t=58847 jiffies, g=58, c=57, q=1258)
  Sending NMI from CPU 4 to CPUs 2:
  CPU 2 didn't respond to backtrace IPI, inspecting paca.
  irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 3623 (bash)
  Back trace of paca->saved_r1 (0xc0000000e1c83ba0) (possibly stale):
  Call Trace:
  [c0000000e1c83ba0] [0000000000000014] 0x14 (unreliable)
  [c0000000e1c83bc0] [c000000000765798] lkdtm_do_action+0x48/0x80
  [c0000000e1c83bf0] [c000000000765a40] direct_entry+0x110/0x1b0
  [c0000000e1c83c90] [c00000000058e650] full_proxy_write+0x90/0xe0
  [c0000000e1c83ce0] [c0000000003aae3c] __vfs_write+0x6c/0x1f0
  [c0000000e1c83d80] [c0000000003ab214] vfs_write+0xd4/0x240
  [c0000000e1c83dd0] [c0000000003ab5cc] ksys_write+0x6c/0x110
  [c0000000e1c83e30] [c00000000000b860] system_call+0x58/0x6c
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>