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tools+docs: the freeze from inside - all 79 threads are in a wait, and two of them burn CPU cycling

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The gdb route works, and the recipe is written down: run-canary execs $XENIA_BIN,
so a wrapper that execs "gdb --args <real binary>" keeps the lockfile, the flags
and the process name (gdb forks and execs the real binary, so ps -C xenia_canary
still finds the inferior) while satisfying ptrace_scope=1 by being the parent.
The Release binary is not stripped - 26595 symtab entries - so frames have names.
The handle SIGSEGV/SIGBUS/SIG32-35 lines are mandatory: xenia uses SIGSEGV for
guest memory watches and the RT signals for thread suspend.

A run froze after ~4 minutes of flight, screen still "flight" rather than GAME
OVER, and all 79 threads had backtraces. EVERY ONE is in a wait - guest threads
in KeWaitForSingleObject / NtWaitForSingleObjectEx / SelfSuspend, the GPU command
processor parked idle, the main thread in poll().

It is nevertheless burning 1253 ticks per 10 s: 403 in the TimerQueue thread
(nanosleep inside TimerThreadMain) and 290 + 274 in two guest threads that the
backtrace shows blocked in KeWaitForSingleObject. A thread genuinely blocked
cannot burn 28% of a core, so those two are CYCLING - a timed wait that expires
and is re-entered - with the timer thread servicing them hot. Three samples
minutes apart show identical frames.

That refines the earlier "the guest is spinning, not deadlocked": the CPU burn is
real but it is in the WAIT PATH inside the kernel layer, not in guest code. The
shape is an event that never gets signalled.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PMRJjbxLqZtsb5Vb7KunPE
This commit is contained in:
Sylpheed RE agent
2026-08-24 01:13:01 +00:00
parent b67f8d5c59
commit 63f74c9c84
3 changed files with 109 additions and 0 deletions
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31
docs/re/captures/stage02-freeze-gdb-backtraces.txt Normal file
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# Backtraces of a FROZEN Stage 02 run, taken with the emulator started
# under gdb (ptrace_scope=1 forbids attaching to a running one).
# 79 threads had backtraces; EVERY ONE was in a wait.
#
# CPU over 10 s while frozen, from /proc/<tid>/stat: 1253 ticks total.
# 403 xenia_canary (the TimerQueue thread)
# 290 XThreadD61F96C0
# 274 XThreadD45FF6C0
# 59 XMA Decoder
Thread 2 (Thread 0x7ffff55326c0 (LWP 103706) "xenia_canary"):
#0 0x00007ffff6699bdf in clock_nanosleep () at /lib/x86_64-linux-gnu/libc.so.6
#1 0x00007ffff66a6b27 in nanosleep () at /lib/x86_64-linux-gnu/libc.so.6
#2 0x0000555555bc07bd in xe::threading::TimerQueue::TimerThreadMain() ()
#3 0x00007ffff69c4db4 in ??? () at /lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x00007ffff6649b84 in ??? () at /lib/x86_64-linux-gnu/libc.so.6
#5 0x00007ffff66d6d6c in ??? () at /lib/x86_64-linux-gnu/libc.so.6
Thread 68 (Thread 0x7ffcd61f96c0 (LWP 103773) "XThreadD61F96C0"):
#0 0x00007ffff6645e51 in ??? () at /lib/x86_64-linux-gnu/libc.so.6
#1 0x00007ffff66488cd in pthread_cond_wait () at /lib/x86_64-linux-gnu/libc.so.6
#2 0x0000555555bbb45b in xe::threading::PosixConditionBase::Wait(std::chrono::duration<long, std::ratio<1l, 1000l> >) ()
#3 0x0000555555e4fd80 in xe::kernel::XObject::Wait(unsigned int, unsigned int, unsigned int, unsigned long*) ()
#4 0x0000555555e313f6 in xe::kernel::shim::ExportRegistrerHelper<(xe::kernel::shim::KernelModuleId)0, (unsigned short)176, xe::kernel::shim::ResultBase<unsigned int>, xe::kernel::shim::PointerParam c
#5 0x00000000a000013a in ??? ()
Thread 69 (Thread 0x7ffcd45ff6c0 (LWP 103774) "XThreadD45FF6C0"):
#0 0x00007ffff6645e51 in ??? () at /lib/x86_64-linux-gnu/libc.so.6
#1 0x00007ffff66488cd in pthread_cond_wait () at /lib/x86_64-linux-gnu/libc.so.6
#2 0x0000555555bbb45b in xe::threading::PosixConditionBase::Wait(std::chrono::duration<long, std::ratio<1l, 1000l> >) ()
#3 0x0000555555e4fd80 in xe::kernel::XObject::Wait(unsigned int, unsigned int, unsigned int, unsigned long*) ()
#4 0x0000555555e313f6 in xe::kernel::shim::ExportRegistrerHelper<(xe::kernel::shim::KernelModuleId)0, (unsigned short)176, xe::kernel::shim::ResultBase<unsigned int>, xe::kernel::shim::PointerParam c
#5 0x00000000a000013a in ??? ()