runtime: skip wakep call in wakeNetPoller on Plan 9

This was part of a performance improvement made by CL 232298 to
reduce timer latency. On multiprocessor Plan 9 machines, it triggers
memory faults often enough that the builder test suite never completes
successfully. See issue #42303 for discussion. As shown by the benchmark
result below, worst case latency on plan9_arm is very bad even with the
wakep call in place - in the tickers-per-P=1 case, a 3ms timer is 270ms late.
Skipping the wakep call and running the benchmark again shows some cases
worse, some better. The performance cost doesn't seem excessive for this
temporary workaround which makes the plan9_arm builders usable again.

With wakep call:

cpu% go test -bench Latency time
goos: plan9
goarch: arm
pkg: time
BenchmarkParallelTimerLatency-4     	     100	  10985859 avg-late-ns	  18630963 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=1-4          	     195	 270294688 avg-late-ns	 542057670 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=2-4          	     234	 182452000 avg-late-ns	 423933688 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=3-4          	     280	 193003004 avg-late-ns	 408034405 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=4-4          	     282	 132819086 avg-late-ns	 313624570 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=5-4          	     339	  71152187 avg-late-ns	 189014519 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=6-4          	     315	  26860484 avg-late-ns	 101759844 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=7-4          	     357	  19106739 avg-late-ns	  59435620 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=8-4          	     376	   7246933 avg-late-ns	  38888461 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=9-4          	     267	  40476892 avg-late-ns	 205851926 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=10-4         	     294	  87836303 avg-late-ns	 252059695 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=2ms/tickers-per-P=1-4            	     379	   4127144 avg-late-ns	  10494927 max-late-ns

Without wakep call:

BenchmarkParallelTimerLatency-4     	      61	  10775151 avg-late-ns	  18668517 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=1-4          	     199	 299587535 avg-late-ns	 597182307 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=2-4          	     272	 184561831 avg-late-ns	 449739837 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=3-4          	     235	 154983257 avg-late-ns	 370940553 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=4-4          	     290	 150034689 avg-late-ns	 332399843 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=5-4          	     298	  47540764 avg-late-ns	 133709031 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=6-4          	     350	  20379394 avg-late-ns	  81742809 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=7-4          	     363	  14403223 avg-late-ns	  98901212 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=8-4          	     375	  12293090 avg-late-ns	  50266552 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=9-4          	     336	  40628820 avg-late-ns	 150946099 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=300µs/tickers-per-P=10-4         	     289	  88265539 avg-late-ns	 280770418 max-late-ns
BenchmarkStaggeredTickerLatency/work-dur=2ms/tickers-per-P=1-4            	     375	   8364937 avg-late-ns	  22598421 max-late-ns

Fixes #42303

Change-Id: I70c63cb2a2bad46950a7cd9dfc7bb32943710d32
Reviewed-on: https://go-review.googlesource.com/c/go/+/275672
Reviewed-by: David du Colombier <0intro@gmail.com>
Trust: Michael Pratt <mpratt@google.com>

src/runtime/proc.go

@@ -2882,7 +2882,9 @@ func wakeNetPoller(when int64) {
 	} else {
 		// There are no threads in the network poller, try to get
 		// one there so it can handle new timers.
-		wakep()
+		if GOOS != "plan9" { // Temporary workaround - see issue #42303.
+			wakep()
+		}
 	}
 }