Thanks. clear now.
在2020年8月7日星期五 UTC+8 上午1:00:51<Ian Lance Taylor> 写道：
> On Thu, Aug 6, 2020 at 7:49 AM HailangGe <everg...@gmail.com> wrote:
> >
> > Recently I was trying to understand how asynchronous preemption is
> implemented in Go 1.14 and
> > basically figured out the call chain.
> >
> > sysmon
> > ↓
> > retake
> > ↓
> > preemptone
> > ↓
> > preemptM
> > ↓
> > signalM(mp, sigPreempt) //send SIGURG to mp
> > ↓
> > doSigPreempt(signal handler) //if isAsyncSafePoint returns true, it will
> go on to preempt
> > ↓
> > ctxt.pushCall(funcPC(asyncPreempt))
> > ↓
> > asyncPreempt
> > ↓
> > asyncPreempt2
> > ↓
> > mcall(preemptPark) or mcall(gopreempt_m)
> >
> > And I wrote a simple program to verify:
> > ```Go
> > func main() {
> > var wg sync.WaitGroup
> > wg.Add(30)
> > for i := 0; i < 30; i++ {
> > go func() {
> > defer wg.Done()
> > t := 0
> > for i := 0; i < 1e8; i++ {
> > t += 2
> > }
> > }()
> > }
> > wg.Wait()
> > }
> > ```
> >
> > As we know general purpose registers RAX/RBX/... are not included in
> goroutine context.
>
> That is true but it doesn't matter. asyncPreempt is a GOARCH-specific
> routine that saves all registers. When the preempted goroutine is
> resumed, it will return through asyncPreempt and the registers will be
> restored before continuing with execution.
>
>
>
>
> > My problem is how Go judges which instruction is preemptable? is it
> determined at compile time?
>
> There are other, less common, cases where preemption is not permitted,
> and, yes, those are marked by the compiler and checked by
> isAsyncSafePoint.
>
> Ian
>
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