X hangs for several seconds periodically when no GL clients are open

[Noah Meyerhans]

Not sure if this is an issue with the X server or with the kernel i915 module, so I'm starting with X.

The system is a Thinkpad X1 Carbon, Gen 7. Details on the graphics hardware (from lspci -vv) are below. BIOS is up-to-date as of Nov. 21.

The symptom is that X will not redraw screen state for several seconds at a time under certain circumstances. Characters typed into a window, or an application that attempts to redraw regularly, are the easiest demonstration of this. E.g. running 'vmstat 1' in a terminal (which should print one line of text every second), I see no activity for roughly 5 seconds, and then all 5 seconds worth of content are printed.

Generating some activity with the mouse/trackpad seems to trigger a refresh before the full time delay expires.

This problem does *not* occur in the default Ubuntu GNOME environment. It appears that it only occurs when there are no GL clients running. In a minimal "failsafe" X session, it is reproducible 100% of the time. In a failsafe session with the "glxdemo" program from the "mesa-utils" package installed, the problem does not occur. I suspect that there's enough GL usage in the standard GNOME configuration that this doesn't affect most users, but my default X session doesn't use much GL.

00:02.0 VGA compatible controller: Intel Corporation UHD Graphics 620 (Whiskey Lake) (rev 02) (prog-if 00 [VGA controller])
        Subsystem: Lenovo Device 2292
        Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
        Latency: 0
        Interrupt: pin A routed to IRQ 130
        Region 0: Memory at e9000000 (64-bit, non-prefetchable) [size=16M]
        Region 2: Memory at c0000000 (64-bit, prefetchable) [size=256M]
        Region 4: I/O ports at 2000 [size=64]
        [virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
        Capabilities: [40] Vendor Specific Information: Len=0c <?>
        Capabilities: [70] Express (v2) Root Complex Integrated Endpoint, MSI 00
                DevCap: MaxPayload 128 bytes, PhantFunc 0
                        ExtTag- RBE+
                DevCtl: Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
                        RlxdOrd- ExtTag- PhantFunc- AuxPwr- NoSnoop-
                        MaxPayload 128 bytes, MaxReadReq 128 bytes
                DevSta: CorrErr- UncorrErr- FatalErr- UnsuppReq- AuxPwr- TransPend-
                DevCap2: Completion Timeout: Not Supported, TimeoutDis-, LTR-, OBFF Not Supported
                DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-, LTR-, OBFF Disabled
        Capabilities: [ac] MSI: Enable+ Count=1/1 Maskable- 64bit-
                Address: fee00018 Data: 0000
        Capabilities: [d0] Power Management version 2
                Flags: PMEClk- DSI+ D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=0 PME-
        Capabilities: [100 v1] Process Address Space ID (PASID)
                PASIDCap: Exec- Priv-, Max PASID Width: 14
                PASIDCtl: Enable- Exec- Priv-
        Capabilities: [200 v1] Address Translation Service (ATS)
                ATSCap: Invalidate Queue Depth: 00
                ATSCtl: Enable-, Smallest Translation Unit: 00
        Capabilities: [300 v1] Page Request Interface (PRI)
                PRICtl: Enable- Reset-
                PRISta: RF- UPRGI- Stopped+
                Page Request Capacity: 00008000, Page Request Allocation: 00000000
        Kernel driver in use: i915
        Kernel modules: i915

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xserver-xorg-core-hwe-18.04 2:1.20.4-1ubuntu3~18.04.1
ProcVersionSignature: Ubuntu 5.3.0-19.20~18.04.2-generic 5.3.1
Uname: Linux 5.3.0-19-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
Date: Thu Nov 21 17:12:15 2019
InstallationDate: Installed on 2019-11-15 (6 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
ProcEnviron:
 TERM=xterm
 PATH=(custom, user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/zsh
SourcePackage: xorg-server-hwe-18.04
UpgradeStatus: No upgrade log present (probably fresh install)

[Timo Aaltonen (tjaalton)]

Does booting with 'i915.enable_psr=0' help?

[Noah Meyerhans (noahm)]

i915.enable_psr=0 does not help with this issue. It does seem to eliminate some artifacts when running in text mode, but that's another story...

[Launchpad Janitor (janitor)]

[Expired for xorg-server-hwe-18.04 (Ubuntu) because there has been no activity for 60 days.]