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

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Message-ID: <20190904062716.GC5541@shbuild999.sh.intel.com>
Date:   Wed, 4 Sep 2019 14:27:16 +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 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?

Thanks,
Feng

> 
> Best regards
> Thomas