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

[Michel Dänzer]

On 2019-07-31 11:25 a.m., Huang, Ying wrote:
> Hi, Daniel,
> 
> Daniel Vetter <daniel at ffwll.ch> writes:
> 
>> 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 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.


-- 
Earthling Michel Dänzer               |              https://www.amd.com
Libre software enthusiast             |             Mesa and X developer