Currently we depend on the good graces and timing of the scheduler to
get opportunities to start dedicated mark workers. In the worst case,
it may take 10ms to get dedicated mark workers going at the beginning
of mark 1 and mark 2 or after the amount of available work has dropped
and gone back up.

Instead of waiting for the regular preemption logic to get around to
us, make putfull enlist a random P if we're not already running enough
dedicated workers. This should improve performance stability of the
garbage collector and is likely to improve the overall performance
somewhat.

No overall effect on the go1 benchmarks. It speeds up the garbage
benchmark by 12%, which more than counters the performance loss from
the previous commit.

name              old time/op  new time/op  delta
XBenchGarbage-12  6.32ms ± 4%  5.58ms ± 2%  -11.68%  (p=0.000 n=20+16)

name                      old time/op    new time/op    delta
BinaryTree17-12              3.18s ± 5%     3.12s ± 4%  -1.83%  (p=0.021 n=20+20)
Fannkuch11-12                2.50s ± 2%     2.46s ± 2%  -1.57%  (p=0.000 n=18+19)
FmtFprintfEmpty-12          50.8ns ± 3%    50.4ns ± 3%    ~     (p=0.184 n=20+20)
FmtFprintfString-12          167ns ± 2%     171ns ± 1%  +2.46%  (p=0.000 n=20+19)
FmtFprintfInt-12             161ns ± 2%     163ns ± 2%  +1.81%  (p=0.000 n=20+20)
FmtFprintfIntInt-12          269ns ± 1%     266ns ± 1%  -0.81%  (p=0.002 n=19+20)
FmtFprintfPrefixedInt-12     237ns ± 2%     231ns ± 2%  -2.86%  (p=0.000 n=20+20)
FmtFprintfFloat-12           313ns ± 2%     313ns ± 1%    ~     (p=0.681 n=20+20)
FmtManyArgs-12              1.05µs ± 2%    1.03µs ± 1%  -2.26%  (p=0.000 n=20+20)
GobDecode-12                8.66ms ± 1%    8.67ms ± 1%    ~     (p=0.380 n=19+20)
GobEncode-12                6.56ms ± 1%    6.56ms ± 2%    ~     (p=0.607 n=19+20)
Gzip-12                      317ms ± 1%     314ms ± 2%  -1.10%  (p=0.000 n=20+19)
Gunzip-12                   42.1ms ± 1%    42.2ms ± 1%  +0.27%  (p=0.044 n=20+19)
HTTPClientServer-12         62.7µs ± 1%    62.0µs ± 1%  -1.04%  (p=0.000 n=19+18)
JSONEncode-12               16.7ms ± 1%    16.8ms ± 2%  +0.59%  (p=0.021 n=20+20)
JSONDecode-12               58.2ms ± 1%    61.4ms ± 2%  +5.43%  (p=0.000 n=18+19)
Mandelbrot200-12            3.84ms ± 1%    3.87ms ± 2%  +0.79%  (p=0.008 n=18+20)
GoParse-12                  3.86ms ± 2%    3.76ms ± 2%  -2.60%  (p=0.000 n=20+20)
RegexpMatchEasy0_32-12       100ns ± 2%     100ns ± 1%  -0.68%  (p=0.005 n=18+15)
RegexpMatchEasy0_1K-12       332ns ± 1%     342ns ± 1%  +3.16%  (p=0.000 n=19+19)
RegexpMatchEasy1_32-12      82.9ns ± 3%    83.0ns ± 2%    ~     (p=0.906 n=19+20)
RegexpMatchEasy1_1K-12       487ns ± 1%     494ns ± 1%  +1.50%  (p=0.000 n=17+20)
RegexpMatchMedium_32-12      131ns ± 2%     130ns ± 1%    ~     (p=0.686 n=19+20)
RegexpMatchMedium_1K-12     39.6µs ± 1%    39.2µs ± 1%  -1.09%  (p=0.000 n=18+19)
RegexpMatchHard_32-12       2.04µs ± 1%    2.04µs ± 2%    ~     (p=0.804 n=20+20)
RegexpMatchHard_1K-12       61.7µs ± 2%    61.3µs ± 2%    ~     (p=0.052 n=18+20)
Revcomp-12                   529ms ± 2%     533ms ± 1%  +0.83%  (p=0.003 n=20+19)
Template-12                 70.7ms ± 2%    71.0ms ± 2%    ~     (p=0.065 n=20+19)
TimeParse-12                 351ns ± 2%     355ns ± 1%  +1.25%  (p=0.000 n=19+20)
TimeFormat-12                362ns ± 2%     373ns ± 1%  +2.83%  (p=0.000 n=18+20)
[Geo mean]                  62.2µs         62.3µs       +0.13%

name                      old speed      new speed      delta
GobDecode-12              88.6MB/s ± 1%  88.5MB/s ± 1%    ~     (p=0.392 n=19+20)
GobEncode-12               117MB/s ± 1%   117MB/s ± 1%    ~     (p=0.622 n=19+20)
Gzip-12                   61.1MB/s ± 1%  61.8MB/s ± 2%  +1.11%  (p=0.000 n=20+19)
Gunzip-12                  461MB/s ± 1%   460MB/s ± 1%  -0.27%  (p=0.044 n=20+19)
JSONEncode-12              116MB/s ± 1%   115MB/s ± 2%  -0.58%  (p=0.022 n=20+20)
JSONDecode-12             33.3MB/s ± 1%  31.6MB/s ± 2%  -5.15%  (p=0.000 n=18+19)
GoParse-12                15.0MB/s ± 2%  15.4MB/s ± 2%  +2.66%  (p=0.000 n=20+20)
RegexpMatchEasy0_32-12     317MB/s ± 2%   319MB/s ± 2%    ~     (p=0.052 n=20+20)
RegexpMatchEasy0_1K-12    3.08GB/s ± 1%  2.99GB/s ± 1%  -3.07%  (p=0.000 n=19+19)
RegexpMatchEasy1_32-12     386MB/s ± 3%   386MB/s ± 2%    ~     (p=0.939 n=19+20)
RegexpMatchEasy1_1K-12    2.10GB/s ± 1%  2.07GB/s ± 1%  -1.46%  (p=0.000 n=17+20)
RegexpMatchMedium_32-12   7.62MB/s ± 2%  7.64MB/s ± 1%    ~     (p=0.702 n=19+20)
RegexpMatchMedium_1K-12   25.9MB/s ± 1%  26.1MB/s ± 2%  +0.99%  (p=0.000 n=18+20)
RegexpMatchHard_32-12     15.7MB/s ± 1%  15.7MB/s ± 2%    ~     (p=0.723 n=20+20)
RegexpMatchHard_1K-12     16.6MB/s ± 2%  16.7MB/s ± 2%    ~     (p=0.052 n=18+20)
Revcomp-12                 481MB/s ± 2%   477MB/s ± 1%  -0.83%  (p=0.003 n=20+19)
Template-12               27.5MB/s ± 2%  27.3MB/s ± 2%    ~     (p=0.062 n=20+19)
[Geo mean]                99.4MB/s       99.1MB/s       -0.35%

Change-Id: I914d8cadded5a230509d118164a4c201601afc06
Reviewed-on: https://go-review.googlesource.com/16298Reviewed-by: Rick Hudson <rlh@golang.org>