goschedImpl transitions the current goroutine from _Grunning to
_Grunnable and places it on the global run queue before calling into
schedule.

It does _not_ call wakep after adding the global run queue. I believe
the intuition behind skipping wakep is that since we are immediately
calling the scheduler so we don't need to wake anything to run this
work. Unfortunately, this intuition is not correct, as it breaks
coordination with spinning Ms [1].

Consider this example scenario:

Initial conditions:

M0: Running P0, G0
M1: Spinning, holding P1 and looking for work

Timeline:

M1: Fails to find work; drops P
M0: newproc adds G1 to P0 runq
M0: does not wakep because there is a spinning M
M1: clear mp.spinning, decrement sched.nmspinning (now in "delicate dance")
M1: check sched.runqsize -> no global runq work
M0: gosched preempts G0; adds G0 to global runq
M0: does not wakep because gosched doesn't wakep
M0: schedules G1 from P0 runq
M1: check P0 runq -> no work
M1: no work -> park

G0 is stranded on the global runq with no M/P looking to run it. This is
a loss of work conservation.

As a result, G0 will have unbounded* scheduling delay, only getting
scheduled when G1 yields. Even once G1 yields, we still won't start
another P, so both G0 and G1 will switch back and forth sharing one P
when they should start another.

*The caveat to this is that today sysmon will preempt G1 after 10ms,
effectively capping the scheduling delay to 10ms, but not solving the P
underutilization problem. Sysmon's behavior here is theoretically
unnecessary, as our work conservation guarantee should allow sysmon to
avoid preemption if there are any idle Ps. Issue #60693 tracks changing
this behavior and the challenges involved.

[1] It would be OK if we unconditionally entered the scheduler as a
spinning M ourselves, as that would require schedule to call wakep when
it finds work in case there is more work.

Fixes #55160.

Change-Id: I2f44001239564b56ea30212553ab557051d22588
Reviewed-on: https://go-review.googlesource.com/c/go/+/501976
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>