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

[Thomas Zimmermann]

Hi

Am 31.07.19 um 10:13 schrieb Daniel Vetter:
> On Tue, Jul 30, 2019 at 10:27 PM Dave Airlie <airlied at gmail.com> wrote:
>>
>> On Wed, 31 Jul 2019 at 05:00, Daniel Vetter <daniel at ffwll.ch> wrote:
>>>
>>> On Tue, Jul 30, 2019 at 8:50 PM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>>>>
>>>> Hi
>>>>
>>>> Am 30.07.19 um 20:12 schrieb Daniel Vetter:
>>>>> On Tue, Jul 30, 2019 at 7:50 PM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>>>>>> Am 29.07.19 um 11:51 schrieb kernel test robot:
>>>>>>> Greeting,
>>>>>>>
>>>>>>> FYI, we noticed a -18.8% regression of vm-scalability.median due to commit:>
>>>>>>>
>>>>>>> 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 vmap/vunmap pair is only executed for fbdev emulation with a shadow
FB. And most of those are with shmem helpers, which ref-count the vmap
calls internally. My guess is that VRAM helpers are currently the only
BOs triggering this problem.

Best regards
Thomas

> -Daniel
> 

-- 
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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