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

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Date:   Wed, 4 Sep 2019 21:15:23 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Feng Tang <feng.tang@...el.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Rong Chen <rong.a.chen@...el.com>,
        Michel Dänzer <michel@...nzer.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Thomas Zimmermann <tzimmermann@...e.de>, LKP <lkp@...org>
Subject: Re: [LKP] [drm/mgag200] 90f479ae51: vm-scalability.median -18.8% regression

On Wed, 4 Sep 2019 at 19:17, Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Wed, Sep 4, 2019 at 10:35 AM Feng Tang <feng.tang@...el.com> wrote:
> >
> > Hi Daniel,
> >
> > On Wed, Sep 04, 2019 at 10:11:11AM +0200, Daniel Vetter wrote:
> > > On Wed, Sep 4, 2019 at 8:53 AM Thomas Zimmermann <tzimmermann@...e.de> wrote:
> > > >
> > > > Hi
> > > >
> > > > Am 04.09.19 um 08:27 schrieb Feng Tang:
> > > > >> 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?
> > > >
> > > > To my knowledge, there's only one instance of the worker. Re-scheduling
> > > > the worker before a previous instance started, will not create a second
> > > > instance. The worker's instance will complete all pending updates. So in
> > > > some way, skipping workers already happens.
> > >
> > > So I think that the most often fbcon update from atomic context is the
> > > blinking cursor. If you disable that one you should be back to the old
> > > performance level I think, since just writing to dmesg is from process
> > > context, so shouldn't change.
> >
> > Hmm, then for the old driver, it should also do the most update in
> > non-atomic context?
> >
> > One other thing is, I profiled that updating a 3MB shadow buffer needs
> > 20 ms, which transfer to 150 MB/s bandwidth. Could it be related with
> > the cache setting of DRM shadow buffer? say the orginal code use a
> > cachable buffer?
>
> Hm, that would indicate the write-combining got broken somewhere. This
> should definitely be faster. Also we shouldn't transfer the hole
> thing, except when scrolling ...

First rule of fbcon usage, you are always effectively scrolling.

Also these devices might be on a PCIE 1x piece of wet string, not sure
if the numbers reflect that.

Dave.