If the scheduler has no user work and there's no GC work visible, it
puts the P to sleep (or blocks on the network). However, if we later
enqueue more GC work, there's currently nothing that specifically
wakes up the scheduler to let it start an idle GC worker. As a result,
we can underutilize the CPU during GC if Ps have been put to sleep.

Fix this by making GC wake idle Ps when work buffers are put on the
full list. We already have a hook to do this, since we use this to
preempt a random P if we need more dedicated workers. We expand this
hook to instead wake an idle P if there is one. The logic we use for
this is identical to the logic used to wake an idle P when we ready a
goroutine.

To make this really sound, we also fix the scheduler to re-check the
idle GC worker condition after releasing its P. This closes a race
where 1) the scheduler checks for idle work and finds none, 2) new
work is enqueued but there are no idle Ps so none are woken, and 3)
the scheduler releases its P.

There is one subtlety here. Currently we call enlistWorker directly
from putfull, but the gcWork is in an inconsistent state in the places
that call putfull. This isn't a problem right now because nothing that
enlistWorker does touches the gcWork, but with the added call to
wakep, it's possible to get a recursive call into the gcWork
(specifically, while write barriers are disallowed, this can do an
allocation, which can dispose a gcWork, which can put a workbuf). To
handle this, we lift the enlistWorker calls up a layer and delay them
until the gcWork is in a consistent state.

Fixes #14179.

Change-Id: Ia2467a52e54c9688c3c1752e1fc00f5b37bbfeeb
Reviewed-on: https://go-review.googlesource.com/32434
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>