CPU spikes and system hangs from gnome-shell every 5-10 seconds

[Radon Rosborough]

On a fresh installation of Ubuntu 20.04 with no configuration files changed yet (the "install third-party software" option enabled), installed from the official daily ISO, the system freezes for about a second every 5-10 seconds and I see a spike to 70% or more CPU usage by gnome-shell. I cannot see anything that shows up in `sudo journalctl -ef` or `sudo dmesg -w` when the freezes occur. The problem reproduces on both X11 and Wayland, although under Wayland it's more annoying since a freeze leads to repeated keypresses of whatever key was being held down on the keyboard at the time of the freeze.

This problem seems to have been reported at <https://www.reddit.com/r/pop_os/comments/gwfn7l/gnome_shell_random_cpu_spikes_every_10_seconds/> but with no plausible resolution or workaround in sight.

I tried to follow <https://superuser.com/q/1046603> to figure out what was going on with gnome-shell, but I don't think the results seem very helpful.

Result of `sudo perf top`:

    4.39% libglib-2.0.so.0.6400.3 [.] g_variant_serialised_n_children
    4.26% libc-2.31.so [.] __vfprintf_internal

And `sudo perf top --sort comm,dso`:

    45.28% gnome-shell libglib-2.0.so.0.6400.3
    17.23% gnome-shell libc-2.31.so

And `sudo perf record -g -p PID` for the `gnome-shell` PID:

    + 39.81% 0.01% [unknown] [.] 0000000000000000
    + 14.26% 2.04% libc-2.31.so [.] __vasprintf_internal
    + 8.61% 8.42% libglib-2.0.so.0.6400.3 [.] g_variant_serialised_n_children
    + 8.51% 8.09% libc-2.31.so [.] __vfprintf_internal

And `strace -c -p PID`:

    % time seconds usecs/call calls errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     34.41 0.128602 5 22784 19837 recvmsg
     25.74 0.096215 7 13716 632 ioctl
     18.04 0.067420 10 6686 poll
      9.50 0.035523 7 4858 write
      4.56 0.017025 7 2322 writev

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gnome-shell 3.36.4-1ubuntu1~20.04.2
ProcVersionSignature: Ubuntu 5.4.0-49.53-generic 5.4.65
Uname: Linux 5.4.0-49-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.10
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 9 07:54:43 2020
DisplayManager: gdm3
InstallationDate: Installed on 2020-10-09 (0 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20201007)
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/usr/bin/zsh
RelatedPackageVersions: mutter-common 3.36.6-1ubuntu0.20.04.2
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)

[Radon Rosborough (raxod502)]

Reattached GsettingsChanges.txt since the apport version contained private information.

[Daniel van Vugt (vanvugt)]

It looks like the freezes could be a separate problem to the CPU spikes. Because these look like input freezes:

[ 6939.689860] shallan gnome-shell[15054]: _clutter_stage_queue_event: assertion 'CLUTTER_IS_STAGE (stage)' failed
[ 6939.689984] shallan gnome-shell[15054]: clutter_do_event: Event does not have a stage: discarding.
[ 6939.699761] shallan gnome-shell[15054]: _clutter_stage_queue_event: assertion 'CLUTTER_IS_STAGE (stage)' failed
[ 6939.699930] shallan gnome-shell[15054]: clutter_do_event: Event does not have a stage: discarding.
[ 6939.729329] shallan gnome-shell[15054]: _clutter_stage_queue_event: assertion 'CLUTTER_IS_STAGE (stage)' failed
[ 6939.729490] shallan gnome-shell[15054]: clutter_do_event: Event does not have a stage: discarding.

but I don't know how that could cause high CPU.

[Daniel van Vugt (vanvugt)]

We will need more information about the affected machine. When the problem is happening please run:

  journalctl -b0 > journal.txt
  lspci -kv > lspci.txt
  xrandr --verbose > xrandr.txt

and then attach the resulting text files here.

[Daniel van Vugt (vanvugt)]

Please also tell us what extensions you have enabled, if any.

[Radon Rosborough (raxod502)]

No extensions were installed aside from whatever came with Ubuntu by default. Unfortunately, I can't provide the requested debugging information at present since I've installed Kubuntu, which does not exhibit the same problem. Hopefully somebody else with the same issue can chime in, however.

[Daniel van Vugt (vanvugt)]

If anyone else experiences similar issues they should open a new bug.