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

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Date:   Mon, 16 Sep 2019 17:06:52 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Rong Chen <rong.a.chen@...el.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>, michel@...nzer.net,
        lkp@...org, linux-kernel@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [LKP] [drm/mgag200] 90f479ae51: vm-scalability.median -18.8%
 regression

Hi Thomas,

On Mon, Sep 09, 2019 at 04:12:37PM +0200, Thomas Zimmermann wrote:
> Hi
> 
> Am 04.09.19 um 08:27 schrieb Feng Tang:
> > Hi Thomas,
> > 
> > On Wed, Aug 28, 2019 at 12:51:40PM +0200, Thomas Zimmermann wrote:
> >> Hi
> >>
> >> Am 28.08.19 um 11:37 schrieb Rong Chen:
> >>> Hi Thomas,
> >>>
> >>> On 8/28/19 1:16 AM, Thomas Zimmermann wrote:
> >>>> Hi
> >>>>
> >>>> Am 27.08.19 um 14:33 schrieb Chen, Rong A:
> >>>>> Both patches have little impact on the performance from our side.
> >>>> Thanks for testing. Too bad they doesn't solve the issue.
> >>>>
> >>>> There's another patch attached. Could you please tests this as well?
> >>>> Thanks a lot!
> >>>>
> >>>> The patch comes from Daniel Vetter after discussing the problem on IRC.
> >>>> The idea of the patch is that the old mgag200 code might display much
> >>>> less frames that the generic code, because mgag200 only prints from
> >>>> non-atomic context. If we simulate this with the generic code, we should
> >>>> see roughly the original performance.
> >>>>
> >>>>
> >>>
> >>> It's cool, the patch "usecansleep.patch" can fix the issue.
> >>
> >> Thank you for testing. But don't get too excited, because the patch
> >> simulates a bug that was present in the original mgag200 code. A
> >> significant number of frames are simply skipped. That is apparently the
> >> reason why it's faster.
> > 
> > Thanks for the detailed info, so the original code skips time-consuming
> > work inside atomic context on purpose. Is there any space to optmise it?
> > If 2 scheduled update worker are handled at almost same time, can one be
> > skipped?
> 
> We discussed ideas on IRC and decided that screen updates could be
> synchronized with vblank intervals. This may give some rate limiting to
> the output.
> 
> If you like, you could try the patch set at [1]. It adds the respective
> code to console and mgag200.

I just tried the 2 patches, no obvious change (comparing to the
18.8% regression), both in overall benchmark and micro-profiling.

90f479ae51afa45e 04a0983095feaee022cdd65e3e4 
---------------- --------------------------- 
     37236 ±  3%      +2.5%      38167 ±  3%  vm-scalability.median
      0.15 ± 24%     -25.1%       0.11 ± 23%  vm-scalability.median_stddev
      0.15 ± 23%     -25.1%       0.11 ± 22%  vm-scalability.stddev
  12767318 ±  4%      +2.5%   13089177 ±  3%  vm-scalability.throughput
 
Thanks,
Feng

> 
> Best regards
> Thomas
> 
> [1]
> https://lists.freedesktop.org/archives/dri-devel/2019-September/234850.html
> 
> > 
> > Thanks,
> > Feng
> > 
> >>
> >> Best regards
> >> Thomas
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
> GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
> HRB 21284 (AG Nürnberg)
>