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

[Chen, Rong A]

Hi Thomas,

On 9/4/2019 4:43 PM, Thomas Zimmermann wrote:
> 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 at suse.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?

There's no obvious change on our system.

Best Regards,
Rong Chen

>
> 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 at lists.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 at lists.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
>
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