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

[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 ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch