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

[Thomas Zimmermann]

Hi

Am 12.06.25 um 01:18 schrieb Michael Kelley:
From: Michael Kelley Sent: Thursday, June 5, 2025 10:39 AM
From: Thomas Zimmermann <tzimmerm...@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-
tzimmerm...@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.

Thanks for the report. It can happen that the vblank timer is handled 
late. It's not strictly an error as the whole thing is best-effort 
anyway. Maybe the timeout could be adjusted.

Best regards
Thomas


Michael


--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)