RE: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers

[Michael Kelley <mhklinux@outlook.com>]

From: Michael Kelley Sent: Thursday, June 5, 2025 10:39 AM
> 
> From: Thomas Zimmermann <tzimmermann@suse.de> Sent: Thursday, June 5, 2025
> 8:36 AM
> >
> > Hi
> >
> > Am 04.06.25 um 23:43 schrieb Michael Kelley:
> > [...]
> > > Nonetheless, there's an underlying issue. A main cause of the difference
> > > is the number of messages to Hyper-V to update dirty regions. With
> > > hyperv_fb using deferred I/O, the messages are limited 20/second, so
> > > the total number of messages to Hyper-V is about 480. But hyperv_drm
> > > appears to send 3 messages to Hyper-V for each line of output, or a total of
> > > about 3,000,000 messages (~90K/second). That's a lot of additional load
> > > on the Hyper-V host, and it adds the 10 seconds of additional elapsed
> > > time seen in the guest. There also this ugly output in dmesg because the
> > > ring buffer for sending messages to the Hyper-V host gets full -- Hyper-V
> > > doesn't always keep up, at least not on my local laptop where I'm
> > > testing:
> > >
> > > [12574.327615] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12574.327684] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12574.327760] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12574.327841] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12597.016128] hyperv_sendpacket: 6211 callbacks suppressed
> > > [12597.016133] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12597.016172] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12597.016220] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > > [12597.016267] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
> > *ERROR* Unable to send packet via vmbus; error -11
> > >
> > > hyperv_drm could be fixed to not output the ugly messages, but there's
> > > still the underlying issue of overrunning the ring buffer, and excessively
> > > hammering on the host. If we could get hyperv_drm doing deferred I/O, I
> > > would feel much better about going full-on with deprecating hyperv_fb.
> >
> > I try to address the problem with the patches at
> >
> > https://lore.kernel.org/dri-devel/20250605152637.98493-1-
> tzimmermann@suse.de/
> >
> > Testing and feedback is much appreciated.
> >
> 
> Nice!
> 
> I ran the same test case with your patches, and everything works well. The
> hyperv_drm numbers are now pretty much the same as the hyperv_fb
> numbers for both elapsed time and system CPU time -- within a few percent.
> For hyperv_drm, there's no longer a gap in the elapsed time and system
> CPU time. No errors due to the guest-to-host ring buffer being full. Total
> messages to Hyper-V for hyperv_drm are now a few hundred instead of 3M.
> The hyperv_drm message count is still a little higher than for hyperv_fb,
> presumably because the simulated vblank rate in hyperv_drm is higher than
> the 20 Hz rate used by hyperv_fb deferred I/O. But the overall numbers are
> small enough that the difference is in the noise. Question: what is the default
> value for the simulated vblank rate? Just curious ...
> 

FYI, I'm seeing this message occasionally when running with your simulated
vblank code and hyperv_drm:

[90920.128278] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm] vblank timer overrun

"Occasionally" is about a dozen occurrences over the last day or so. I can't
yet correlate to any particular activity in the VM. The graphics console has
not been very busy.

Michael