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

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

I was traveling and could reply earlier. Sorry for taking so long.

Am 13.08.19 um 11:36 schrieb Feng Tang:
> Hi Thomas, 
> 
> On Mon, Aug 12, 2019 at 03:25:45PM +0800, Feng Tang wrote:
>> Hi Thomas,
>>
>> On Fri, Aug 09, 2019 at 04:12:29PM +0800, Rong Chen wrote:
>>> Hi,
>>>
>>>>> Actually we run the benchmark as a background process, do we need to
>>>>> disable the cursor and test again?
>>>> There's a worker thread that updates the display from the shadow buffer.
>>>> The blinking cursor periodically triggers the worker thread, but the
>>>> actual update is just the size of one character.
>>>>
>>>> The point of the test without output is to see if the regression comes
>>> >from the buffer update (i.e., the memcpy from shadow buffer to VRAM), or
>>> >from the worker thread. If the regression goes away after disabling the
>>>> blinking cursor, then the worker thread is the problem. If it already
>>>> goes away if there's simply no output from the test, the screen update
>>>> is the problem. On my machine I have to disable the blinking cursor, so
>>>> I think the worker causes the performance drop.
>>>
>>> We disabled redirecting stdout/stderr to /dev/kmsg,  and the regression is
>>> gone.
>>>
>>> commit:
>>>   f1f8555dfb9 drm/bochs: Use shadow buffer for bochs framebuffer console
>>>   90f479ae51a drm/mgag200: Replace struct mga_fbdev with generic framebuffer
>>> emulation
>>>
>>> f1f8555dfb9a70a2  90f479ae51afa45efab97afdde testcase/testparams/testbox
>>> ----------------  -------------------------- ---------------------------
>>>          %stddev      change         %stddev
>>>              \          |                \
>>>      43785                       44481
>>> vm-scalability/300s-8T-anon-cow-seq-hugetlb/lkp-knm01
>>>      43785                       44481        GEO-MEAN vm-scalability.median
>>
>> Till now, from Rong's tests:
>> 1. Disabling cursor blinking doesn't cure the regression.
>> 2. Disabling printint test results to console can workaround the
>> regression.
>>
>> Also if we set the perfer_shadown to 0, the regression is also
>> gone.
> 
> We also did some further break down for the time consumed by the
> new code.
> 
> The drm_fb_helper_dirty_work() calls sequentially 
> 1. drm_client_buffer_vmap	  (290 us)
> 2. drm_fb_helper_dirty_blit_real  (19240 us)
> 3. helper->fb->funcs->dirty()    ---> NULL for mgag200 driver
> 4. drm_client_buffer_vunmap       (215 us)
>

It's somewhat different to what I observed, but maybe I just couldn't
reproduce the problem correctly.

> The average run time is listed after the function names.
> 
> From it, we can see drm_fb_helper_dirty_blit_real() takes too long
> time (about 20ms for each run). I guess this is the root cause
> of this regression, as the original code doesn't use this dirty worker.

True, the original code uses a temporary buffer, but updates the display
immediately.

My guess is that this could be a caching problem. The worker runs on a
different CPU, which doesn't have the shadow buffer in cache.

> As said in last email, setting the prefer_shadow to 0 can avoid
> the regrssion. Could it be an option?

Unfortunately not. Without the shadow buffer, the console's display
buffer permanently resides in video memory. It consumes significant
amount of that memory (say 8 MiB out of 16 MiB). That doesn't leave
enough room for anything else.

The best option is to not print to the console.

Best regards
Thomas

> Thanks,
> Feng
> 
>>
>> --- a/drivers/gpu/drm/mgag200/mgag200_main.c
>> +++ b/drivers/gpu/drm/mgag200/mgag200_main.c
>> @@ -167,7 +167,7 @@ int mgag200_driver_load(struct drm_device *dev, unsigned long flags)
>>  		dev->mode_config.preferred_depth = 16;
>>  	else
>>  		dev->mode_config.preferred_depth = 32;
>> -	dev->mode_config.prefer_shadow = 1;
>> +	dev->mode_config.prefer_shadow = 0;
>>
>> And from the perf data, one obvious difference is good case don't
>> call drm_fb_helper_dirty_work(), while bad case calls.
>>
>> Thanks,
>> Feng
>>
>>> Best Regards,
>>> Rong Chen
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> 

-- 
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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