runtime/trace: preempted background mark worker is started with GoStart, not GoStartLabel

[dominikh]

Normally when a runtime.gcBgMarkWorker goroutine starts running, it is started with GoStartLabel, which allows differentiating between dedicated and idle workers:

36951793 GoBlock p=14 g=13 off=257842
36954632 GoStartLabel p=15 g=13 off=11040855 g=13 seq=5 labelid=2 label=GC (dedicated)

However, when such a goroutine was preempted before it starts running again, GoStart is used:

364170789 GoPreempt p=4 g=13 off=222
364173647 GoStart p=4 g=13 off=227 g=13 seq=0

The trace consumer can work around this, but more consistency would be nice, if it isn't too difficult.

/cc @mknyszek

[mknyszek]

I'll keep this in mind when working on #60773.

[mknyszek]

I'm pretty sure this is a consequence of the GoStartLocal event path which doesn't re-emit the label. This is indeed fixed in the new tracer, because the GoStartLocal optimization doesn't exist anymore (for better or for worse). It doesn't show up as GoStartLocal because the old trace parser rewrites all GoStartLocal events as GoStart events.

Unfortunately this isn't a super simple fix in the old tracer. Either we always emit GoStart for this case or we change the format of GoStartLocal. The former is certainly the easier one, but I'm inclined to just not fix it. Please comment if you disagree.