Eli Zaretskii <eliz@gnu.org> writes:
> Can you attach a debugger to Emacs when it is wedged, and see which
> threads run and consume CPU?

Does below include what you're looking for?  If not, just let me know and I'll investigate further.

Joseph

GNU gdb (GDB) 7.10 [GDB v7.10 for FreeBSD]
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Type "apropos word" to search for commands related to "word".
Attaching to process 57303
Reading symbols from /usr/home/jrm/scm/nm/emacs.git/src/emacs-25.0.50.1...done.
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[New Thread 16b5c00 (LWP 101370)]
[New Thread 1902400 (LWP 100742)]
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[Switching to Thread 1902400 (LWP 100742)]
0x0000000807444cfa in _sigprocmask () from /lib/libc.so.7
SIGINT is used by the debugger.
Are you sure you want to change it? (y or n) [answered Y; input not from terminal]
DISPLAY = :0.0
TERM = screen-256color
Breakpoint 1 at 0x5dfb54: file emacs.c, line 371.
Temporary breakpoint 2 at 0x61e9d9: file sysdep.c, line 905.
(gdb)