Bug 219570 - Reading from vivid results in 100% CPU use
Summary: Reading from vivid results in 100% CPU use
Status: RESOLVED PATCH_ALREADY_AVAILABLE
Alias: None
Product: v4l-dvb
Classification: Unclassified
Component: v4l-core (show other bugs)
Hardware: AMD Linux
: P3 low
Assignee: v4l-dvb_v4l-core@kernel-bugs.osdl.org
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2024-12-07 04:06 UTC by Artem S. Tashkinov
Modified: 2024-12-09 15:03 UTC (History)
1 user (show)

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Regression: No
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Description Artem S. Tashkinov 2024-12-07 04:06:41 UTC 
I'm not sure this command should result in a 100% CPU use:

ffmpeg -i /dev/video0 -f rawvideo -pixel_format yuv420p -video_size 640x360 -c:v libx264 -preset ultrafast -crf 10 /tmp/result.mkv

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
235941  root      20                           R 100.0         0:11.53 vivid-000-vid-cap

There's no ffmpeg in the top because it compresses so fast, it can be barely registered by top.

This is reproducible in 6.12.2.

My CPU is Ryzen 7 5800X and it's plenty fast. I'm almost sure reading from vivid must not result in any perceivable CPU use since it's generating a static picture.

I could be totally wrong though, please advise.
Comment 1 Artem S. Tashkinov 2024-12-07 04:07:15 UTC 
Hans, please take a look.
Comment 2 Hans Verkuil 2024-12-09 14:28:35 UTC 
When waiting in the kthread until it is time to create the next frame, vivid uses:

while (!it_is_time)
    schedule()

Which is correct, but inefficient. If no other CPU activity is ongoing, then this is equivalent to a busy loop.

I'll replace it with a wait_event_interruptible_timeout().
Comment 3 Artem S. Tashkinov 2024-12-09 15:03:26 UTC 
Thanks a lot for the patch!

Fixed by: https://lore.kernel.org/linux-media/f7bc5105-332f-414b-8160-3c0ff679c608@xs4all.nl/T/#u
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