CL 36792 fixed #17953, a linear scan caused by n goroutines piling into
two different locks that hashed to the same bucket in the semaphore table.
In that CL, n goroutines contending for 2 unfortunately chosen locks
went from O(n²) to O(n).

This CL fixes a different linear scan, when n goroutines are contending for
n/2 different locks that all hash to the same bucket in the semaphore table.
In this CL, n goroutines contending for n/2 unfortunately chosen locks
goes from O(n²) to O(n log n). This case is much less likely, but any linear
scan eventually hurts, so we might as well fix it while the problem is fresh
in our minds.

The new test in this CL checks for both linear scans.

The effect of this CL on the sync benchmarks is negligible
(but it fixes the new test).

name                      old time/op    new time/op    delta
Cond1-48                     576ns ±10%     575ns ±13%     ~     (p=0.679 n=71+71)
Cond2-48                    1.59µs ± 8%    1.61µs ± 9%     ~     (p=0.107 n=73+69)
Cond4-48                    4.56µs ± 7%    4.55µs ± 7%     ~     (p=0.670 n=74+72)
Cond8-48                    9.87µs ± 9%    9.90µs ± 7%     ~     (p=0.507 n=69+73)
Cond16-48                   20.4µs ± 7%    20.4µs ±10%     ~     (p=0.588 n=69+71)
Cond32-48                   45.4µs ±10%    45.4µs ±14%     ~     (p=0.944 n=73+73)
UncontendedSemaphore-48     19.7ns ±12%    19.7ns ± 8%     ~     (p=0.589 n=65+63)
ContendedSemaphore-48       55.4ns ±26%    54.9ns ±32%     ~     (p=0.441 n=75+75)
MutexUncontended-48         0.63ns ± 0%    0.63ns ± 0%     ~     (all equal)
Mutex-48                     210ns ± 6%     213ns ±10%   +1.30%  (p=0.035 n=70+74)
MutexSlack-48                210ns ± 7%     211ns ± 9%     ~     (p=0.184 n=71+72)
MutexWork-48                 299ns ± 5%     300ns ± 5%     ~     (p=0.678 n=73+75)
MutexWorkSlack-48            302ns ± 6%     300ns ± 5%     ~     (p=0.149 n=74+72)
MutexNoSpin-48               135ns ± 6%     135ns ±10%     ~     (p=0.788 n=67+75)
MutexSpin-48                 693ns ± 5%     689ns ± 6%     ~     (p=0.092 n=65+74)
Once-48                     0.22ns ±25%    0.22ns ±24%     ~     (p=0.882 n=74+73)
Pool-48                     5.88ns ±36%    5.79ns ±24%     ~     (p=0.655 n=69+69)
PoolOverflow-48             4.79µs ±18%    4.87µs ±20%     ~     (p=0.233 n=75+75)
SemaUncontended-48          0.80ns ± 1%    0.82ns ± 8%   +2.46%  (p=0.000 n=60+74)
SemaSyntNonblock-48          103ns ± 4%     102ns ± 5%   -1.11%  (p=0.003 n=75+75)
SemaSyntBlock-48             104ns ± 4%     104ns ± 5%     ~     (p=0.231 n=71+75)
SemaWorkNonblock-48          128ns ± 4%     129ns ± 6%   +1.51%  (p=0.000 n=63+75)
SemaWorkBlock-48             129ns ± 8%     130ns ± 7%     ~     (p=0.072 n=75+74)
RWMutexUncontended-48       2.35ns ± 1%    2.35ns ± 0%     ~     (p=0.144 n=70+55)
RWMutexWrite100-48           139ns ±18%     141ns ±21%     ~     (p=0.071 n=75+73)
RWMutexWrite10-48            145ns ± 9%     145ns ± 8%     ~     (p=0.553 n=75+75)
RWMutexWorkWrite100-48       297ns ±13%     297ns ±15%     ~     (p=0.519 n=75+74)
RWMutexWorkWrite10-48        588ns ± 7%     585ns ± 5%     ~     (p=0.173 n=73+70)
WaitGroupUncontended-48     0.87ns ± 0%    0.87ns ± 0%     ~     (all equal)
WaitGroupAddDone-48         63.2ns ± 4%    62.7ns ± 4%   -0.82%  (p=0.027 n=72+75)
WaitGroupAddDoneWork-48      109ns ± 5%     109ns ± 4%     ~     (p=0.233 n=75+75)
WaitGroupWait-48            0.17ns ± 0%    0.16ns ±16%   -8.55%  (p=0.000 n=56+75)
WaitGroupWaitWork-48        1.78ns ± 1%    2.08ns ± 5%  +16.92%  (p=0.000 n=74+70)
WaitGroupActuallyWait-48    52.0ns ± 3%    50.6ns ± 5%   -2.70%  (p=0.000 n=71+69)

https://perf.golang.org/search?q=upload:20170215.1

Change-Id: Ia29a8bd006c089e401ec4297c3038cca656bcd0a
Reviewed-on: https://go-review.googlesource.com/37103
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>