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 ... Michael
