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

Thu Aug 1 09:59:28 UTC 2019

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?

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
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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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