This reduces pause time by ~25% relative to tip and by ~50% relative
to Go 1.5.1.

Currently one of the steps of STW mark termination is to loop (in
parallel) over all spans to find objects with finalizers in order to
mark all objects reachable from these objects and to treat the
finalizer special as a root. Unfortunately, even if there are no
finalizers at all, this loop takes roughly 1 ms/heap GB/core, so
multi-gigabyte heaps can quickly push our STW time past 10ms.

Fix this by moving this scan from mark termination to concurrent scan,
where it can run in parallel with mutators. The loop itself could also
be optimized, but this cost is small compared to concurrent marking.

Making this scan concurrent introduces two complications:

1) The scan currently walks the specials list of each span without
locking it, which is safe only with the world stopped. We fix this by
speculatively checking if a span has any specials (the vast majority
won't) and then locking the specials list only if there are specials
to check.

2) An object can have a finalizer set after concurrent scan, in which
case it won't have been marked appropriately by concurrent scan. If
the finalizer is a closure and is only reachable from the special, it
could be swept before it is run. Likewise, if the object is not marked
yet when the finalizer is set and then becomes unreachable before it
is marked, other objects reachable only from it may be swept before
the finalizer function is run. We fix this issue by making
addfinalizer ensure the same marking invariants as markroot does.

For multi-gigabyte heaps, this reduces max pause time by 20%–30%
relative to tip (depending on GOMAXPROCS) and by ~50% relative to Go
1.5.1 (where this loop was neither concurrent nor parallel). Here are
the results for the garbage benchmark:

               ---------------- max pause ----------------
Heap   Procs   Concurrent scan   STW parallel scan   1.5.1
24GB     12         18ms              23ms            37ms
24GB      4         18ms              25ms            37ms
 4GB      4         3.8ms            4.9ms           6.9ms

In all cases, 95%ile pause time is similar to the max pause time. This
also improves mean STW time by 10%–30%.

Fixes #11485.

Change-Id: I9359d8c3d120a51d23d924b52bf853a1299b1dfd
Reviewed-on: https://go-review.googlesource.com/14982Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>