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

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Date:   Wed, 4 Sep 2019 10:43:40 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Feng Tang <feng.tang@...el.com>, Daniel Vetter <daniel@...ll.ch>
Cc:     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>, LKP <lkp@...org>
Subject: Re: [LKP] [drm/mgag200] 90f479ae51: vm-scalability.median -18.8%
 regression

Hi

Am 04.09.19 um 10:35 schrieb Feng Tang:
> 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?
> 
> 
>>
>> https://unix.stackexchange.com/questions/3759/how-to-stop-cursor-from-blinking
>>
>> Bunch of tricks, but tbh I haven't tested them.
> 
> Thomas has suggested to disable curson by
> 	echo 0 > /sys/devices/virtual/graphics/fbcon/cursor_blink
> 
> We tried that way, and no change for the performance data.

There are several ways of disabling the cursor. On my test system, I entered

  tput civis

before the test and got better performance. Did you try this as well?

Best regards
Thomas

> 
> Thanks,
> Feng
> 
>>
>> In any case, I still strongly advice you don't print anything to dmesg
>> or fbcon while benchmarking, because dmesg/printf are anything but
>> fast, especially if a gpu driver is involved. There's some efforts to
>> make the dmesg/printk side less painful (untangling the console_lock
>> from printk), but fundamentally printing to the gpu from the kernel
>> through dmesg/fbcon won't be cheap. It's just not something we
>> optimize beyond "make sure it works for emergencies".
>> -Daniel
>>
>>>
>>> Best regards
>>> Thomas
>>>
>>>>
>>>> Thanks,
>>>> Feng
>>>>
>>>>>
>>>>> Best regards
>>>>> Thomas
>>>> _______________________________________________
>>>> dri-devel mailing list
>>>> dri-devel@...ts.freedesktop.org
>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>>
>>>
>>> --
>>> Thomas Zimmermann
>>> Graphics Driver Developer
>>> SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
>>> GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
>>> HRB 21284 (AG Nürnberg)
>>>
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel@...ts.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>
>>
>>
>> -- 
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
HRB 21284 (AG Nürnberg)

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