Quoting Sean Paul (2019-10-10 23:48:08)
From: Sean Paul seanpaul@chromium.org
*Record scratch* You read that subject correctly, I bet you're wondering how we got here. At least hear me out before you flame :-)
For a long while now, we (ChromeOS) have been struggling getting any value out of user feedback reports of display failures (notably external displays not working). The problem is that all logging, even fatal errors (well, fatal in the sense that a display won't light up) are logged at DEBUG log level. So in order to extract these logs, you need to be able to turn on logging, and reproduce the issue with debug enabled. Unfortunately, this isn't really something we can ask CrOS users I spoke with airlied about this and RHEL has similar issues.
This is the point where you ask me, "So Sean, why don't you just enable DRM_UT_BLAH?". Good question! Here are the reasons in ascending order of severity: 1- People aren't consistent with their categories, so we'd have to enable a bunch to get proper coverage 2- We don't want to overwhelm syslog with drm spam, others have to use it too 3- Console logging is slow
Hopefully you're with me so far. I have a problem that has no existing solution. What I really want is a ringbuffer of the most recent logs (in the categories I'm interested in) exposed via debugfs so I can extract it when users file feedback.
A nitpick, tracepoints are no longer in debugfs but tracefs. I'm being told the reason is because production environments want to use them and expect them to be stable.
It just so happens that there is something which does _exactly_ this! I can dump the most recent logs into tracepoints, turn them on and off depending on which category I want, and pull them from debugfs on demand.
"What about trace_printk()?" You'll say. It doesn't give us the control we get from using tracepoints and it's not meant to be left sprinkled around in code.
So that is how we got here, now it's time for you to tell me why this is a horrible idea :-)
Being devil's advocate; How long until all our debugging messages will be kernel ABI?
In the context of the DRM subsystem level unified tracing (Message-Id: 20190121232040.26114-1-chris@chris-wilson.co.uk) we already struggled to find the sweet spot of exposing only information we can maintain long term.
I can imagine this de-railing into to a direction when the userspace debugging information of interest is extracted from the kernel debug messages. When that message format changes and breaks the userspace tool, you probably know the drill.
I like the base idea, but implementation through tracepoints has great potential to become maintenance nightmare. So maybe something actually in debugfs might be the right solution?
Regards, Joonas