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

Thomas Zimmermann
2025-06-12T07:25:08Z

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 <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.

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
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