Currently scanstack obtains its own gcWork from the P for the duration
of the stack scan and then, if called during mark termination,
disposes the gcWork.

However, this means that the number of workbufs allocated will be at
least the number of stacks scanned during mark termination, which may
be very high (especially during a STW GC). This happens because, in
steady state, each scanstack will obtain a fresh workbuf (either from
the empty list or by allocating it), fill it with the scan results,
and then dispose it to the full list. Nothing is consuming from the
full list during this (and hence nothing is recycling them to the
empty list), so the length of the full list by the time mark
termination starts draining it is at least the number of stacks
scanned.

Fix this by pushing the gcWork acquisition up the stack to either the
gcDrain that calls markroot that calls scanstack (which batches across
many stack scans and is the path taken during STW GC) or to newstack
(which is still a single scanstack call, but this is roughly bounded
by the number of Ps).

This fix reduces the workbuf allocation for the test program from
issue #15319 from 213 MB (roughly 2KB * 1e5 goroutines) to 10 MB.

Fixes #15319.

Note that there's potentially a similar issue in write barriers during
mark 2. Fixing that will be more difficult since there's no broader
non-preemptible context, but it should also be less of a problem since
the full list is being drained during mark 2.

Some overall improvements in the go1 benchmarks, plus the usual noise.
No significant change in the garbage benchmark (time/op or GC memory).

name                      old time/op    new time/op    delta
BinaryTree17-12              2.54s ± 1%     2.51s ± 1%  -1.09%  (p=0.000 n=20+19)
Fannkuch11-12                2.12s ± 0%     2.17s ± 0%  +2.18%  (p=0.000 n=19+18)
FmtFprintfEmpty-12          45.1ns ± 1%    45.2ns ± 0%    ~     (p=0.078 n=19+18)
FmtFprintfString-12          127ns ± 0%     128ns ± 0%  +1.08%  (p=0.000 n=19+16)
FmtFprintfInt-12             125ns ± 0%     122ns ± 1%  -2.71%  (p=0.000 n=14+18)
FmtFprintfIntInt-12          196ns ± 0%     190ns ± 1%  -2.91%  (p=0.000 n=12+20)
FmtFprintfPrefixedInt-12     196ns ± 0%     194ns ± 1%  -0.94%  (p=0.000 n=13+18)
FmtFprintfFloat-12           253ns ± 1%     251ns ± 1%  -0.86%  (p=0.000 n=19+20)
FmtManyArgs-12               807ns ± 1%     784ns ± 1%  -2.85%  (p=0.000 n=20+20)
GobDecode-12                7.13ms ± 1%    7.12ms ± 1%    ~     (p=0.351 n=19+20)
GobEncode-12                5.89ms ± 0%    5.95ms ± 0%  +0.94%  (p=0.000 n=19+19)
Gzip-12                      219ms ± 1%     221ms ± 1%  +1.35%  (p=0.000 n=18+20)
Gunzip-12                   37.5ms ± 1%    37.4ms ± 0%    ~     (p=0.057 n=20+19)
HTTPClientServer-12         81.4µs ± 4%    81.9µs ± 3%    ~     (p=0.118 n=17+18)
JSONEncode-12               15.7ms ± 1%    15.8ms ± 1%  +0.73%  (p=0.000 n=17+18)
JSONDecode-12               57.9ms ± 1%    57.2ms ± 1%  -1.34%  (p=0.000 n=19+19)
Mandelbrot200-12            4.12ms ± 1%    4.10ms ± 0%  -0.33%  (p=0.000 n=19+17)
GoParse-12                  3.22ms ± 2%    3.25ms ± 1%  +0.72%  (p=0.000 n=18+20)
RegexpMatchEasy0_32-12      70.6ns ± 1%    71.1ns ± 2%  +0.63%  (p=0.005 n=19+20)
RegexpMatchEasy0_1K-12       240ns ± 0%     239ns ± 1%  -0.59%  (p=0.000 n=19+20)
RegexpMatchEasy1_32-12      71.3ns ± 1%    71.3ns ± 1%    ~     (p=0.844 n=17+17)
RegexpMatchEasy1_1K-12       384ns ± 2%     371ns ± 1%  -3.45%  (p=0.000 n=19+20)
RegexpMatchMedium_32-12      109ns ± 1%     108ns ± 2%  -0.48%  (p=0.029 n=19+19)
RegexpMatchMedium_1K-12     34.3µs ± 1%    34.5µs ± 2%    ~     (p=0.160 n=18+20)
RegexpMatchHard_32-12       1.79µs ± 9%    1.72µs ± 2%  -3.83%  (p=0.000 n=19+19)
RegexpMatchHard_1K-12       53.3µs ± 4%    51.8µs ± 1%  -2.82%  (p=0.000 n=19+20)
Revcomp-12                   386ms ± 0%     388ms ± 0%  +0.72%  (p=0.000 n=17+20)
Template-12                 62.9ms ± 1%    62.5ms ± 1%  -0.57%  (p=0.010 n=18+19)
TimeParse-12                 325ns ± 0%     331ns ± 0%  +1.84%  (p=0.000 n=18+19)
TimeFormat-12                338ns ± 0%     343ns ± 0%  +1.34%  (p=0.000 n=18+20)
[Geo mean]                  52.7µs         52.5µs       -0.42%

Change-Id: Ib2d34736c4ae2ec329605b0fbc44636038d8d018
Reviewed-on: https://go-review.googlesource.com/23391
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>