[Bug 107421] New: [HSW][bisected] "sna: Defer submission of the next shadow frame until halfway through" causes stuttering

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107421

            Bug ID: 107421
           Summary: [HSW][bisected] "sna: Defer submission of the next
                    shadow frame until halfway through" causes stuttering
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: thomas.lindroth at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Hardware: Haswell i7-4790K IGPU using all 3 connectors. All 3 monitors run at
1920x1200 at 59.95hz

Software: kernel-4.14.59, xorg-server-1.19.5, xf86-video-intel-git
(AccelMethod=sna, TearFree=on), mesa-17.3.9, xfce-4.12 without compositing.

commit af36a4ab78cc0e2a85fa40d442bfb92df75dd217
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Sun Apr 1 13:13:19 2018 +0100

    sna: Defer submission of the next shadow frame until halfway through

    Do not immediately post the next shadow flip on completion of the
    current flips, but instead queue a timer for half way through the next
    vblank so that try to we keep the additional input-output lag to less
    than a frame.

After commit af36a4ab78cc0e2a85fa40d442bfb92df75dd217 I've been getting a lot
of stuttering when playing video. Both mpv and glxgears stutters. By stutter I
mean dropped and duplicated frames.

The stutter only happens when multiple heads are active. Either clone mode or
side by side. The stutter is periodic. It happens every 20 sec and last for
about 5 sec. The remaining 15 sec is smooth. My guess is that I just ended up
with those times on my system by accident. A side effect of my framerate and
how fast the framerate drifts between my monitors or something like that.

I assumed the problem was in kernel space first and while debugging I noticed
that the tracepoint i915:intel_engine_notify ring=0 fires a lot during
stuttering but not when playback is smooth. notify_ring() is part of the irq
handler so the RCS engine sends interrupts during stutter but not otherwise. I
don't know what that means. The commit before the bad commit have no periodic
stutter but the RCS engine sends interrupts almost constantly instead.

Let me know if you need any special debug output.

-- 
You are receiving this mail because:
You are the QA Contact for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/intel-gfx-bugs/attachments/20180729/9cb5f15f/attachment.html>