Ubuntu 18.04 drains battery very fast after wake up from a deep sleep

Short answer: It turned out to be a problem somewhere between kernel 4.15 and the touchpad.

Disabling the touchpad solved the problem:

sudo rmmod hid_multitouch

This is an acceptable workaround for me, since most of the time I use an external mouse. In rare cases, when I don't have a mouse at hand, I solve the problem by disabling + enabling the touchpad after waking up:

sudo rmmod hid_multitouch
sudo modprobe hid_multitouch

EDIT March 2020: Some time ago, I've realized that a moment of using the touchpad, after resuming the laptop also solves the problem.

Long answer

The top command showed a low overall CPU usage (below 8%), but surprisingly the following command:

watch -n1 "cat /proc/cpuinfo | grep \"^[c]pu MHz\""

revealed that all CPU cores were running at the highest frequency (~3.8 GHz)!, while I was expecting to see the lowest possible frequency (0.8 GHz) due to the idle state. The sudo powertop has proven that the high power consumption indeed was coming from the CPU.

So I came back to the results of the top command, watched it closely, and one process looked suspicious: irq/51-SYNA2393:

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
21874 root      20   0       0      0      0 I   2.0  0.0   0:03.18 kworker/0:0
    1 root      20   0  225780   9392   6472 S   1.0  0.0   0:21.78 systemd
  513 root     -51   0       0      0      0 D   5.3  0.0   1:00.15 irq/51-SYNA2393
 2673 greg      20   0 4569240 205392  73916 S   1.0  0.6   1:54.94 gnome-shell
 ...

The dmesg | grep SYNA output:

[148428.423272] input: SYNA2393:00 06CB:7A13 Touchpad (...)
[148428.423893] hid-multitouch 0018:06CB:7A13.0001: input,hidraw1: I2C HID v1.00 Mouse [SYNA2393:00 06CB:7A13] on i2c-SYNA2393:00

revealed that IRQ 51 comes from the laptop touchpad. That's how I found the culprit.

To be honest, I'm not sure it it's a bug inside Linux kernel, or simply my touchpad is not top-notch supported. When I'll try a newer Ubuntu version with newer kernel I'll update this post.