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

[Huang, Ying]

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.

Best Regards,
Huang, Ying