On Fri, 23 Aug 2019 at 03:25, Thomas Zimmermann tzimmermann@suse.de wrote:
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 44481vm-scalability/300s-8T-anon-cow-seq-hugetlb/lkp-knm01 43785 44481 GEO-MEAN vm-scalability.median
Till now, from Rong's tests:
- Disabling cursor blinking doesn't cure the regression.
- 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
- drm_client_buffer_vmap (290 us)
- drm_fb_helper_dirty_blit_real (19240 us)
- helper->fb->funcs->dirty() ---> NULL for mgag200 driver
- 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.
Wait a second, I thought the driver did an eviction on modeset of the scanned out object, this was a deliberate design decision made when writing those drivers, has this been removed in favour of gem and generic code paths?
Dave.