From: Thomas Zimmermann <tzimmerm...@suse.de> Sent: Tuesday, September 16, 2025 1:31 AM > > Hi > > Am 09.09.25 um 05:29 schrieb Michael Kelley: > > From: Michael Kelley Sent: Thursday, September 4, 2025 10:36 PM > >> From: Thomas Zimmermann <tzimmerm...@suse.de> Sent: Thursday, September 4, > >> 2025 7:56 AM > >>> Compositors often depend on vblanks to limit their display-update > >>> rate. Without, they see vblank events ASAP, which breaks the rate- > >>> limit feature. This creates high CPU overhead. It is especially a > >>> problem with virtual devices with fast framebuffer access. > >>> > >>> The series moves vkms' vblank timer to DRM and converts the hyperv > >>> DRM driver. An earlier version of this series contains examples of > >>> other updated drivers. In principle, any DRM driver without vblank > >>> hardware can use the timer. > >> I've tested this patch set in a Hyper-V guest against the > >> linux-next20250829 > >> kernel. All looks good. Results and perf are the same as reported here [4]. > >> So far I haven't seen the "vblank timer overrun" error, which is consistent > >> with the changes you made since my earlier testing. I'll keep running this > >> test kernel for a while to see if anything anomalous occurs. > > As I continued to run with this patch set, I got a single occurrence of this > > WARN_ON. I can't associate it with any particular action as I didn't notice > > it until well after it occurred. > > I've investigated. The stack trace comes from the kernel console's > display update. It runs concurrently to the vblank timeout. What likely > happens here is that the update code reads two values and in between, > the vblank timeout updates them. So the update then compares an old and > a new value; leading to an incorrect result with triggers the warning. > > I'll include a fix in the series' next iteration. But I also think that > it's not critical. DRM's vblank helpers can usually deal with such problems.
Thanks! I'm giving your v4 series a try now. Good that the underlying problem is not critical. But I was seeing the WARN_ON() output in dmesg every few days (a total of 4 times now), and that's not really acceptable even if everything continues to work correctly. Michael
