sim: runtime debug flags toggle via SIGRTMIN+1#3181
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Add support for toggling gem5 debug flags in a running simulation without restarting. A SIGRTMIN+1 handler (debugCmdHandler) sets async_debug_cmd; doSimLoop() dispatches processDebugCmd() which reads /tmp/gem5_debug_<pid>.cmd and applies the commands. Command format (one per line, # comments supported): +FLAG enable flag (stays on until disabled) +FLAG:Nt enable flag, auto-disable after N ticks -FLAG disable flag immediately trace:on global trace master enable trace:off global trace master disable SIGRTMIN is reserved for KVM (KVM_KICK_SIGNAL in cpu/kvm/base.hh), so SIGRTMIN+1 is used to avoid conflicts. Auto-disable is implemented via DebugFlagOffEvent, a self-deleting Event scheduled on the main event queue. Unknown flag names produce a warning but do not abort processing of remaining commands. util/gem5_debug_ctl.sh is a companion shell script that writes the command file and sends SIGRTMIN+1 (kill -RTMIN+1) to the target pid. base/debug.hh: Flag::_tracing is made std::atomic<bool> with memory_order_relaxed to eliminate the data race between the main thread calling changeFlag() and subordinate threads reading _tracing in DPRINTFs. On x86/ARM64 relaxed atomics compile to plain loads/stores so there is no performance overhead.
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Follows up on discussion #1598.
Summary
This patch adds the ability to enable and disable gem5 debug flags in a
running simulation without restarting it.
gem5 installs a handler on
SIGRTMIN+1at simulation start. A companionshell script (
util/gem5_debug_ctl.sh) writes commands to a temporary fileand sends the signal. Commands are processed on the main event loop at the
next sim tick after delivery — no mid-instruction races.
Supported commands:
+FLAG+FLAG:Nt-FLAGtrace:ontrace:offExample usage:
Confirmation lines (
gem5_debug_ctl: ...) appear in the sim's stdout/log.Design notes
SIGRTMIN+1is used to avoidSIGRTMINwhich is reserved for KVM(
KVM_KICK_SIGNAL).async_event+async_debug_cmd, wakes the primary event queue, returnsimmediately. All file I/O and flag changes happen on the main thread.
Flag::_tracingis changed frombooltostd::atomic<bool>(relaxedordering) to eliminate the data race that arises when
changeFlag()iscalled while other host threads are executing
DPRINTF.Event(AutoDelete)scheduled at
curTick() + duration.This implementation was developed with AI assistance.