基于信号的抢占式调度实现原理

// sigPreempt is the signal used for non-cooperative preemption.
//
// There's no good way to choose this signal, but there are some
// heuristics:
//
// 1. It should be a signal that's passed-through by debuggers by
// default. On Linux, this is SIGALRM, SIGURG, SIGCHLD, SIGIO,
// SIGVTALRM, SIGPROF, and SIGWINCH, plus some glibc-internal signals.
//
// 2. It shouldn't be used internally by libc in mixed Go/C binaries
// because libc may assume it's the only thing that can handle these
// signals. For example SIGCANCEL or SIGSETXID.
//
// 3. It should be a signal that can happen spuriously without
// consequences. For example, SIGALRM is a bad choice because the
// signal handler can't tell if it was caused by the real process
// alarm or not (arguably this means the signal is broken, but I
// digress). SIGUSR1 and SIGUSR2 are also bad because those are often
// used in meaningful ways by applications.
//
// 4. We need to deal with platforms without real-time signals (like
// macOS), so those are out.
//
// We use SIGURG because it meets all of these criteria, is extremely
// unlikely to be used by an application for its "real" meaning (both
// because out-of-band data is basically unused and because SIGURG
// doesn't report which socket has the condition, making it pretty
// useless), and even if it is, the application has to be ready for
// spurious SIGURG. SIGIO wouldn't be a bad choice either, but is more
// likely to be used for real.
const sigPreempt = _SIGURG