[LKP] [drm/mgag200] 90f479ae51: vm-scalability.median -18.8% regression

Thomas Zimmermann tzimmermann at suse.de
Thu Aug 1 11:58:38 UTC 2019


Am 01.08.19 um 13:25 schrieb Feng Tang:
> Hi Thomas,
> On Thu, Aug 01, 2019 at 11:59:28AM +0200, Thomas Zimmermann wrote:
>> 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 <tzimmermann at suse.de>
>>> 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?
> Sure, please send it to us. Rong and I will try it.

Fantastic, thank you! The patch set is available on dri-devel at


Best regards

> Thanks,
> Feng
>> 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 at 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)
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> dri-devel at lists.freedesktop.org
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Thomas Zimmermann
Graphics Driver Developer
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
HRB 21284 (AG Nürnberg)

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