[Nexus 6P Q&A, Help & Troubleshooting] XCnathan32:
Seems that a voltage issue may be causing the problem, while doing some more testing, I found these lines in this log:

[ 3.651551] qcom,qpnp-power-on qpnp-power-on-2: PMIC@SID0 Power-on reason: Triggered from PON1 (secondary PMIC) and 'cold' boot

[ 3.651573] qcom,qpnp-power-on qpnp-power-on-2: PMIC@SID0: Power-off reason: Triggered from PS_HOLD (PS_HOLD/MSM controlled shutdown)

[ 3.651773] input: qpnp_pon as /devices/virtual/input/input4

[ 3.651832] [sweep2wake]: set device qpnp_pon

[ 3.652624] pon_spare_reg: no parameters

[ 3.652711] qcom,qpnp-power-on qpnp-power-on-12: No PON config. specified

[ 3.652745] qcom,qpnp-power-on qpnp-power-on-12: PMIC@SID2 Power-on reason: Triggered from PON1 (secondary PMIC) and 'cold' boot

[ 3.652761] qcom,qpnp-power-on qpnp-power-on-12: PMIC@SID2: Unknown power-off reason

(lines 1090-1097)

PMIC= Power management integrated circuit "A PMIC may include battery management, voltage regulation, and charging functions. It may include a DC to DC converter to allow dynamic voltage scaling. Some models are known to feature up to 95% power conversion efficiency. Some models integrate with dynamic frequency scaling in a combination known as DVFS (dynamic voltage and frequency scaling)." Source: Wikipedia.

The PMIC may be the culprit, because power-on-2 seems to work fine, but power-on 12 seems to fail, so if 2 and 12 correspond to the little and BIG clusters, it would explain why the little cores work but not the big. If the problem is a bad PMIC, it all seems to make sense, bad PMIC=random battery problems and CPU not getting power, resulting in system unresponsiveness.

Unfortunately, if I'm right, there's probably no software fix to this (that gets all 8 cores working), and everyone's Nexus is probably ticking down to the point where the PMIC fails on the little cores. I'm gonna research the PMIC on the 6P, and see if the PMIC model matches other devices with this error.

Edit: So after looking at a couple other logs, they show that power-on-12 shuts down with "Power-off reason: Triggered from PS_HOLD (PS_HOLD/MSM controlled shutdown)" so maybe the unknown shutdown isn't a problem, but after looking at the logs for more voltage issues, I found these: "mem_acc0_corner: 0 <--> 0 mV" I've seen this in literally every log I've pulled (and in the past few days I've literally pulled hundreds) That line occurs multiple times, and for different devices as well, notably "mem_acc1_corner" So that could mean that some components aren't getting power.

Also, every single time before a watchdog bite, this is the line that precedes the watchdog errors "SMBCHG: smbchg_external_power_changed: smbchg_external_power_changed: current_limit = 0" Also sounds like a power issue. Someone correct me if I'm wrong, but I really think I'm onto something here.

Edit 2: Nexus 6p, 5x, G4, V10 (all known for being very problematic devices), all have the qualcomm PMI8994 PMIC, I don't know if this is common to all 808/810 devices. There's also a facebook post here: https://www.facebook.com/LGUSAMobile/posts/901868279848418, where someone claims his G4 had to be repaired twice, both times for PMIC. His problem was different (screen glitches and overheating) but still, very suspicious and coincidental that all of these factors would come together.

Edit 3: New PMIC8994's sell for about 2 bucks, unfortunately, replacing them involves soldering onto a BGA. I do remember watching videos on how to solder a BGA, except it was for an eMMC, I might consider buying a new PMIC to test my theory.