[Bug 214923] kqueue hangs with busy loop

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214923

Konstantin Belousov <kib at FreeBSD.org> changed:

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                 CC|                            |kib at FreeBSD.org

--- Comment #1 from Konstantin Belousov <kib at FreeBSD.org> ---
(In reply to yjh0502 from comment #0)
I do not see a clean indicator that the process 'spins on the kqueue syscall'. 
The wait channel demonstrates that the process waits for some event subsystem
mutex (the star before the name), but due to the truncated output, I am not
sure which mutex is it.  It could kqueue lock, or kqueue global lock, for
instance.

That said, WCPU 100% indicates that there is some other thread in the process
which consumes cycles, which is consistent with the report of four threads in
the process.

>From that data, I am not even convinced that this is a kernel bug and not some
application issue.  For very beginning, look at the kernel backtraces of all
threads in your process with procstat -kk.  Find and backtrace the lock owner
of a kqueue lock.  All that assuming that you believe that the issue is a
kernel problem.  Otherwise, and I would start with that route, would I debug it
myself, is to look at the app and see what it is doing (wrong).

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