snapd generating wakeup events, keeps CPU from being idle

I have been finding snapd wakes up at ~12Hz, putting it among the top 10 kernel tasks and processes consuming energy under PowerTOP on an idle GNOME desktop, only GNOME terminal open.

I have been seeing the same behaviour since snapd was mandated; given no housekeeping is expected, these wakeups can most likely be avoided. On Ubuntu 21.10 on a Dell XPS 15:

PowerTOP v2.11 Overview Idle stats Frequency stats Device stats Tunables WakeUp

The battery reports a discharge rate of 5.27 W
The power consumed was 111 J
The estimated remaining time is 15 hours, 0 minutes

Summary: 157.6 wakeups/second, 0.0 GPU ops/seconds, 0.0 VFS ops/sec and 2.2% CPU use

Power est. Usage Events/s Category Description
...
 49.6 mW 4.2 ms/s 4.7 Process [PID 1966] /usr/bin/gnome-shell
 49.6 mW 194.8 µs/s 13.3 Process [PID 13] [rcu_sched]
 42.4 mW 135.1 µs/s 11.5 kWork psi_avgs_work
 42.2 mW 105.6 µs/s 11.5 Process [PID 888] /usr/lib/snapd/snapd

strace while no snaps are open shows frequent messages being read from fd 4:

$ sudo strace -ffp $(pidof snapd)
[pid 5860] futex(0x55d25953e338, FUTEX_WAIT_PRIVATE, 0, {tv_sec=34, tv_nsec=89621} <unfinished ...>
[pid 5899] <... select resumed>) = 1 (in [4])
[pid 5899] futex(0x55d25953e338, FUTEX_WAKE_PRIVATE, 1) = 1
[pid 5899] recvfrom(4, <unfinished ...>
[pid 5860] <... futex resumed>) = 0
[pid 5899] <... recvfrom resumed>[{prefix="libudev", magic=htonl(0xfeedcafe), header_size=40, properties_off=40, properties_len=370, filter_subsystem_hash=htonl(0xc370b302), filter_devtype_hash=htonl(0), filter_tag_bloom_hi=htonl(0x2080400), filter_tag_bloom_lo=htonl(0x10c00001)}, "ACTION=change\0DEVPATH=/devices/p"...], 4096, MSG_PEEK, {sa_family=AF_NETLINK, nl_pid=429, nl_groups=0x000002}, [112 => 12]) = 410
[pid 5860] nanosleep({tv_sec=0, tv_nsec=20000}, <unfinished ...>
[pid 5899] recvfrom(4, [{prefix="libudev", magic=htonl(0xfeedcafe), header_size=40, properties_off=40, properties_len=370, filter_subsystem_hash=htonl(0xc370b302), filter_devtype_hash=htonl(0), filter_tag_bloom_hi=htonl(0x2080400), filter_tag_bloom_lo=htonl(0x10c00001)}, "ACTION=change\0DEVPATH=/devices/p"...], 4096, 0, {sa_family=AF_NETLINK, nl_pid=429, nl_groups=0x000002}, [112 => 12]) = 410
[pid 5860] <... nanosleep resumed>NULL) = 0
[pid 5899] fcntl(10, F_GETFL) = 0 (flags O_RDONLY)
[pid 5899] fcntl(10, F_SETFL, O_RDONLY) = 0
[pid 5899] select(11, [4 10], NULL, NULL, NULL <unfinished ...>
[pid 5860] nanosleep({tv_sec=0, tv_nsec=20000}, NULL) = 0
[pid 5860] futex(0x55d25953e338, FUTEX_WAIT_PRIVATE, 0, {tv_sec=25, tv_nsec=990469943}

(and similar, repeating)

File descriptor 4 corresponds to a kernel netlink socket:

$ lsof -p $(pidof snapd)
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
...
snapd 5859 root 4u netlink 0t0 68137 KOBJECT_UEVENT

For this Dell XPS 9510, there is an ambient light sensor, which continuously adjusts the screen brightness via DBus - these events are not being filtered by snapd; the filtering should be improved accordingly.

@watchmaker any chance you could run `udevadm monitor` and collect the log with events?

snapd monitors udev device changes with filter set SUBSYSTEM == "usb" || SUBSYSTEM == "tty" || SUBSYSTEM == "net".