异步抢占:死循环的 goroutine 怎么被信号打断

  main 每次 time.Sleep(10ms) 实际耗时:最小 25.2 ms / 中位 30.0 ms / 最大 30.1 ms(30 次,GOMAXPROCS=1,GODEBUG="")
  (超时:15 s 内没有结束,被我杀掉了;GOMAXPROCS=1,GODEBUG='asyncpreemptoff=1')

  死循环 goroutine 在跑时调用 runtime.GC():耗时 0.2 ms(GOMAXPROCS=2,GODEBUG="")
  (超时:4 s 内没有结束,被我杀掉了;GOMAXPROCS=2,GODEBUG='asyncpreemptoff=1')

  死循环的 goroutine 占着唯一的 P 跑了约 1 秒:收到 SIGURG 19 次(GOMAXPROCS=1,GODEBUG="")
  (超时:4 s 内没有结束,被我杀掉了;GOMAXPROCS=1,GODEBUG='asyncpreemptoff=1')

  main.spinNoCall STEXT size=48 align=0x0 args=0x0 locals=0x0 funcid=0x0 leaf
    (<gtmp>/ap2/main.go:16)	TEXT	main.spinNoCall(SB), LEAF|NOFRAME|ABIInternal, $0-0
    (<gtmp>/ap2/main.go:16)	FUNCDATA	$0, gclocals·g5+hNtRBP6YXNjfog7aZjQ==(SB)
    (<gtmp>/ap2/main.go:16)	FUNCDATA	$1, gclocals·g5+hNtRBP6YXNjfog7aZjQ==(SB)
    (<gtmp>/ap2/main.go:19)	MOVD	ZR, R0
    (<gtmp>/ap2/main.go:20)	ADD	$1, R0, R0
    (<gtmp>/ap2/main.go:21)	TST	$268435455, R0
    (<gtmp>/ap2/main.go:21)	BNE	4
    (<unknown line number>)	NOP
    (<GOROOT>/src/sync/atomic/type.go:19)	MOVD	$main.stop(SB), R1
    (<GOROOT>/src/sync/atomic/type.go:19)	LDARW	(R1), R1
    (<gtmp>/ap2/main.go:21)	CBZW	R1, 4
    (<gtmp>/ap2/main.go:21)	RET	(R30)
    0x0000 e0 03 1f aa 00 04 00 91 1f 6c 40 f2 c1 ff ff 54  .........l@....T
    0x0010 01 00 00 90 21 00 00 91 21 fc df 88 41 ff ff 34  ....!...!...A..4
  这个函数的汇编里 CALL 指令的个数:0(上面是整个函数:LEAF|NOFRAME,没有栈检查的序言,也没有任何调用)

  preempt.go:7-22
    // A goroutine can be preempted at any safe-point. Currently, there
    // are a few categories of safe-points:
    //
    // 1. A blocked safe-point occurs for the duration that a goroutine is
    //    descheduled, blocked on synchronization, or in a system call.
    //
    // 2. Synchronous safe-points occur when a running goroutine checks
    //    for a preemption request.
    //
    // 3. Asynchronous safe-points occur at any instruction in user code
    //    where the goroutine can be safely paused and a conservative
    //    stack and register scan can find stack roots. The runtime can
    //    stop a goroutine at an async safe-point using a signal.
    //
    // At both blocked and synchronous safe-points, a goroutine's CPU
    // state is minimal and the garbage collector has complete information
  proc.go:6677-6679
    // forcePreemptNS is the time slice given to a G before it is
    // preempted.
    const forcePreemptNS = 10 * 1000 * 1000 // 10ms
  extern.go:227-229
    	asyncpreemptoff: asyncpreemptoff=1 disables signal-based
    	asynchronous goroutine preemption. This makes some loops
    	non-preemptible for long periods, which may delay GC and
  signal_unix.go:44-45
    // sigPreempt is the signal used for non-cooperative preemption.
    //