Hi Am 01.08.19 um 10:37 schrieb Feng Tang: > On Thu, Aug 01, 2019 at 02:19:53PM +0800, Rong Chen wrote: >>>>>>>>>>> >>>>>>>>>>> commit: 90f479ae51afa45efab97afdde9b94b9660dd3e4 ("drm/mgag200: Replace struct mga_fbdev with generic framebuffer emulation") >>>>>>>>>>> https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next.git master >>>>>>>>>> Daniel, Noralf, we may have to revert this patch. >>>>>>>>>> >>>>>>>>>> I expected some change in display performance, but not in VM. Since it's >>>>>>>>>> a server chipset, probably no one cares much about display performance. >>>>>>>>>> So that seemed like a good trade-off for re-using shared code. >>>>>>>>>> >>>>>>>>>> Part of the patch set is that the generic fb emulation now maps and >>>>>>>>>> unmaps the fbdev BO when updating the screen. I guess that's the cause >>>>>>>>>> of the performance regression. And it should be visible with other >>>>>>>>>> drivers as well if they use a shadow FB for fbdev emulation. >>>>>>>>> For fbcon we should need to do any maps/unamps at all, this is for the >>>>>>>>> fbdev mmap support only. If the testcase mentioned here tests fbdev >>>>>>>>> mmap handling it's pretty badly misnamed :-) And as long as you don't >>>>>>>>> have an fbdev mmap there shouldn't be any impact at all. >>>>>>>> The ast and mgag200 have only a few MiB of VRAM, so we have to get the >>>>>>>> fbdev BO out if it's not being displayed. If not being mapped, it can be >>>>>>>> evicted and make room for X, etc. >>>>>>>> >>>>>>>> To make this work, the BO's memory is mapped and unmapped in >>>>>>>> drm_fb_helper_dirty_work() before being updated from the shadow FB. [1] >>>>>>>> That fbdev mapping is established on each screen update, more or less. >>>>>>>> From my (yet unverified) understanding, this causes the performance >>>>>>>> regression in the VM code. >>>>>>>> >>>>>>>> The original code in mgag200 used to kmap the fbdev BO while it's being >>>>>>>> displayed; [2] and the drawing code only mapped it when necessary (i.e., >>>>>>>> not being display). [3] >>>>>>> Hm yeah, this vmap/vunmap is going to be pretty bad. We indeed should >>>>>>> cache this. >>>>>>> >>>>>>>> I think this could be added for VRAM helpers as well, but it's still a >>>>>>>> workaround and non-VRAM drivers might also run into such a performance >>>>>>>> regression if they use the fbdev's shadow fb. >>>>>>> Yeah agreed, fbdev emulation should try to cache the vmap. >>>>>>> >>>>>>>> Noralf mentioned that there are plans for other DRM clients besides the >>>>>>>> console. They would as well run into similar problems. >>>>>>>> >>>>>>>>>> The thing is that we'd need another generic fbdev emulation for ast and >>>>>>>>>> mgag200 that handles this issue properly. >>>>>>>>> Yeah I dont think we want to jump the gun here. If you can try to >>>>>>>>> repro locally and profile where we're wasting cpu time I hope that >>>>>>>>> should sched a light what's going wrong here. >>>>>>>> I don't have much time ATM and I'm not even officially at work until >>>>>>>> late Aug. I'd send you the revert and investigate later. I agree that >>>>>>>> using generic fbdev emulation would be preferable. >>>>>>> Still not sure that's the right thing to do really. Yes it's a >>>>>>> regression, but vm testcases shouldn run a single line of fbcon or drm >>>>>>> code. So why this is impacted so heavily by a silly drm change is very >>>>>>> confusing to me. We might be papering over a deeper and much more >>>>>>> serious issue ... >>>>>> It's a regression, the right thing is to revert first and then work >>>>>> out the right thing to do. >>>>> Sure, but I have no idea whether the testcase is doing something >>>>> reasonable. If it's accidentally testing vm scalability of fbdev and >>>>> there's no one else doing something this pointless, then it's not a >>>>> real bug. Plus I think we're shooting the messenger here. >>>>> >>>>>> It's likely the test runs on the console and printfs stuff out while running. >>>>> But why did we not regress the world if a few prints on the console >>>>> have such a huge impact? We didn't get an entire stream of mails about >>>>> breaking stuff ... >>>> The regression seems not related to the commit. But we have retested >>>> and confirmed the regression. Hard to understand what happens. >>> Does the regressed test cause any output on console while it's >>> measuring? If so, it's probably accidentally measuring fbcon/DRM code in >>> addition to the workload it's trying to measure. >>> >> >> Sorry, I'm not familiar with DRM, we enabled the console to output logs, and >> attached please find the log file. >> >> "Command line: ... console=tty0 earlyprintk=ttyS0,115200 >> console=ttyS0,115200 vga=normal rw" > > We did more check, and found this test machine does use the > mgag200 driver. > > And we are suspecting the regression is caused by > > commit cf1ca9aeb930df074bb5bbcde55f935fec04e529 > Author: Thomas Zimmermann > Date: Wed Jul 3 09:58:24 2019 +0200 Yes, that's the commit. Unfortunately reverting it would require reverting a hand full of other patches as well. I have a potential fix for the problem. Could you run and verify that it resolves the problem? Best regards Thomas > > drm/fb-helper: Map DRM client buffer only when required > > This patch changes DRM clients to not map the buffer by default. The > buffer, like any buffer object, should be mapped and unmapped when > needed. > > An unmapped buffer object can be evicted to system memory and does > not consume video ram until displayed. This allows to use generic fbdev > emulation with drivers for low-memory devices, such as ast and mgag200. > > This change affects the generic framebuffer console. HW-based consoles > map their console buffer once and keep it mapped. Userspace can mmap this > buffer into its address space. The shadow-buffered framebuffer console > only needs the buffer object to be mapped during updates. While not being > updated from the shadow buffer, the buffer object can remain unmapped. > Userspace will always mmap the shadow buffer. > > which may add more load when fbcon is busy printing out messages. > > We are doing more test inside 0day to confirm. > > Thanks, > Feng > _______________________________________________ > dri-devel mailing list > dri-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel > -- Thomas Zimmermann Graphics Driver Developer SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany GF: Felix Imendörffer, Mary Higgins, Sri Rasiah HRB 21284 (AG Nürnberg)