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

[Feng Tang]

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.

--- 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