Power On/Off Scheduling


Some time ago, I applied for a job where I had to implement a tool to schedule power on and power off on a computer.

The target was to have a file where to define a weekly schedule. There also should be a software to parse that file and act in consequence. It was also needed to add exceptions on certain days, specified by month and month day, where the schedule has to be different from the matching day of week. So the file should look something like this:

Monday = 8.30-20.00
Tuesday = 8.30-13.15
Wednesday = 8.30-
Thursday = -20.00
Friday = 8.30-19.00
Saturday =
Sunday = 8.30-20.00

Nov_25 = 8.30-22.00

Sadly, the company that was hiring me tricked me with the price so we never achieved an agreement (or at least that's what I deduced from having no answer to my mails). However, as they needed it urgently, I was implementing the software during the negotiations so that I had finished my job when thy stopped answering me. So, as I thought it is an interesting software and there are many people that unknown how to do schedule a power on, I decided to post it in my blog.

Let's get to the point. This tool was coded in python and it has 3 pieces:

As you can see, all the interesting magic will be in necromancer.py file, so let's see how we can schedule the killing of our computer and then get it back from the Avernus.

First we'll see the easy thing: how to schedule the shutdown. Of course the easiest way of doing this is to create a cron task. So when we schedule a power off, necromancer.py will create a file in /etc/cron.d/necromancer_poweroff which will call shutdown and then it will erase itself.

How do we schedule the power on? Using the rtc timer of course. To check if your computer supports rtc, you just have to check if /proc/driver/rtc file exists. The rtc clock is the clock we configure in our BIOS, is the hardware clock, so the first thing we have to check is whether the rtc clock and the system clock have the same date and time (our computer could be using NTP so the hardware clock could be completely different from system time). Also is important to care about the time zone so that hwclock usually is set to UTC and the system time is calculated using it's time zone.

To solve this problem, the firs thing that necromancer.py does is to set the hardware clock to UTC based on the system time. This way we can ensure that if we work in UTC, the time will be always the same in system and rtc.

Now we can schedule a power on date. This can be done by writing the date into the file /sys/class/rtc/rtc0/wakealarm. That date must be in UTC time. We can test with these commands:

# echo 0 > /sys/class/rtc/rtc0/wakealarm

# date -d "1:25:00 Dec 22, 2012" +%s >/sys/class/rtc/rtc0/wakealarm

# cat /proc/driver/rtc
rtc_time : 12:56:37
rtc_date : 2011-12-09
alrm_time : 00:25:00
alrm_date : 2012-12-22
alarm_IRQ : yes
alrm_pending : no
update IRQ enabled : no
periodic IRQ enabled : no
periodic IRQ frequency : 1024
max user IRQ frequency : 64
24hr : yes
periodic_IRQ : no
update_IRQ : no
HPET_emulated : yes
BCD : yes
DST_enable : no
periodic_freq : 1024
batt_status : okay

The first line resets the rtc wake alarm. We set the wake alarm in the second line (note the -u parameter to use UTC). In the third line, we see the lines alrm_time and alrm_date. As you can see, the alarm is established for the date we specified. I'm in UTC+1 so the time is one hour less to be in UTC.

All this is done in necromancer.py. But, how the software works altogether? Well, It's simple, once you have changed the schedule in the configuration file stored in /etc/power_scheduler.cfg by default, the powerscheduler.py tool has to be called. This can be done calling the script from cron each five minutes or so but I recomment to use icron as the best solution. I'll talk about icron in other post some day.

Well, thats all for now. All the files used here are stored in https://github.com/diego-XA/dgtool/tree/master/powerscheduler. Feel free to use the tool and modify it.