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

Fri Aug 2 09:11:50 UTC 2019

On Wed, Jul 31, 2019 at 12:10:54PM +0200, Thomas Zimmermann wrote:
> 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.

I meant that surely this vm-scalability testcase isn't the only thing
that's being run by 0day on a machine with mga200g. If a few printks to
dmesg/console cause such a huge regression, I'd expect everything to
regress on that box. But seems to not be the case.
-Daniel

> 
> 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)
> 


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch