协程调度时机三:抢占式调度

static uint32 retake(int64 now)
{
 uint32 i, s, n;
 int64 t;
 P *p;
 Pdesc *pd;
 n = 0;
 for(i = 0; i < runtime·gomaxprocs; i++) {
 p = runtime·allp[i];
 if(p==nil)
 continue;
 pd = &pdesc[i];
 s = p->status;
 if(s == Psyscall) {
 ......
 } else if(s == Prunning) {
 // Preempt G if it's running for more than 10ms. 
 t = p->schedtick;
 if(pd->schedtick != t) {
 pd->schedtick = t;
 pd->schedwhen = now;
 continue;
 }
 if(pd->schedwhen + 10*1000*1000 > now)
 continue;
 // 如果自从上次发生调度时间已经超过了10ms 
 preemptone(p);
 }
 }
 return n;
}
// 这里的抢占只是将g的preempt设置为true 
// 只有在g进行函数调用时才会检查该标志位 
// 并进而可能发生调度,非常弱 
static bool preemptone(P *p)
{
 M *mp;
 G *gp;
 mp = p->m;
 if(mp == nil || mp == g->m)
 return false;
 gp = mp->curg;
 if(gp == nil || gp == mp->g0)
 return false;
 gp->preempt = true;
 // Every call in a go routine checks for stack overflow by 
 // comparing the current stack pointer to gp->stackguard0. 
 // Setting gp->stackguard0 to StackPreempt folds 
 // preemption into the normal stack overflow check. 
 gp->stackguard0 = StackPreempt;
 return true;
}

TEXT runtime·morestack(SB),NOSPLIT,0ドル-0 
 // Cannot grow scheduler stack (m->g0).
 get_tls(CX)
 MOVQ g(CX), BX 
 MOVQ g_m(BX), BX 
 MOVQ m_g0(BX), SI 
 CMPQ g(CX), SI 
 JNE 2(PC)
 INT 3ドル 
 // Cannot grow signal stack (m->gsignal).
 MOVQ m_gsignal(BX), SI 
 CMPQ g(CX), SI 
 JNE 2(PC)
 INT 3ドル 
 // Called from f.
 // Set m->morebuf to f's caller.
 MOVQ 8(SP), AX // f's caller's PC
 MOVQ AX, (m_morebuf+gobuf_pc)(BX)
 LEAQ 16(SP), AX // f's caller's SP
 MOVQ AX, (m_morebuf+gobuf_sp)(BX)
 get_tls(CX)
 MOVQ g(CX), SI
 MOVQ SI, (m_morebuf+gobuf_g)(BX)
 // Set g->sched to context in f.
 MOVQ 0(SP), AX // f's PC
 MOVQ AX, (g_sched+gobuf_pc)(SI)
 MOVQ SI, (g_sched+gobuf_g)(SI)
 LEAQ 8(SP), AX // f's SP
 MOVQ AX, (g_sched+gobuf_sp)(SI)
 MOVQ DX, (g_sched+gobuf_ctxt)(SI)
 MOVQ BP, (g_sched+gobuf_bp)(SI)
 // Call newstack on m->g0's stack.
 MOVQ m_g0(BX), BX 
 MOVQ BX, g(CX)
 MOVQ (g_sched+gobuf_sp)(BX), SP 
 CALL runtime·newstack(SB)
 MOVQ 0,ドル 0x1003 // crash if newstack returns
 RET 

func newstack() {
 thisg := getg()
 // TODO: double check all gp. shouldn't be getg().
 if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
 throw("stack growth after fork")
 }
 if thisg.m.morebuf.g.ptr() != thisg.m.curg {
 print("runtime: newstack called from g=", thisg.m.morebuf.g, "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")
 morebuf := thisg.m.morebuf 
 traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
 throw("runtime: wrong goroutine in newstack")
 }
 gp := thisg.m.curg 
 morebuf := thisg.m.morebuf 
 thisg.m.morebuf.pc = 0
 thisg.m.morebuf.lr = 0
 thisg.m.morebuf.sp = 0
 thisg.m.morebuf.g = 0
 rewindmorestack(&gp.sched)
 // NOTE: stackguard0 may change underfoot, if another thread
 // is about to try to preempt gp. Read it just once and use that same
 // value now and below.
 preempt := atomicloaduintptr(&gp.stackguard0) == stackPreempt
 if preempt {
 if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.preemptoff != "" || thisg.m.p.ptr().status != _Prunning {
 // Let the goroutine keep running for now.
 // gp->preempt is set, so it will be preempted next time.
 gp.stackguard0 = gp.stack.lo + _StackGuard
 gogo(&gp.sched) // never return
 }
 }
 ......
 // 进行重新调度
 if preempt {
 if gp == thisg.m.g0 {
 throw("runtime: preempt g0")
 }
 if thisg.m.p == 0 && thisg.m.locks == 0 {
 throw("runtime: g is running but p is not")
 }
 if gp.preemptscan {
 for !castogscanstatus(gp, _Gwaiting, _Gscanwaiting) {
 // Likely to be racing with the GC as
 // it sees a _Gwaiting and does the
 // stack scan. If so, gcworkdone will
 // be set and gcphasework will simply
 // return.
 }
 if !gp.gcscandone {
 scanstack(gp)
 gp.gcscandone = true
 }
 gp.preemptscan = false
 gp.preempt = false
 casfrom_Gscanstatus(gp, _Gscanwaiting, _Gwaiting)
 casgstatus(gp, _Gwaiting, _Grunning)
 gp.stackguard0 = gp.stack.lo + _StackGuard
 gogo(&gp.sched) // never return
 }
 // Act like goroutine called runtime.Gosched.
 casgstatus(gp, _Gwaiting, _Grunning)
 // 放弃当前协程,调度新协程执行
 gopreempt_m(gp) // never return
 }
}

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