runtime: make stack re-scan O(# dirty stacks)
Currently the stack re-scan during mark termination is O(# stacks) because we enqueue a root marking job for every goroutine. It takes ~34ns to process this root marking job for a valid (clean) stack, so at around 300k goroutines we exceed the 10ms pause goal. A non-trivial portion of this time is spent simply taking the cache miss to check the gcscanvalid flag, so simply optimizing the path that handles clean stacks can only improve this so much. Fix this by keeping an explicit list of goroutines with dirty stacks that need to be rescanned. When a goroutine first transitions to running after a stack scan and marks its stack dirty, it adds itself to this list. We enqueue root marking jobs only for the goroutines in this list, so this improves stack re-scanning asymptotically by completely eliminating time spent on clean goroutines. This reduces mark termination time for 500k idle goroutines from 15ms to 238µs. Overall performance effect is negligible. name \ 95%ile-time/markTerm old new delta IdleGs/gs:500000/gomaxprocs:12 15000µs ± 0% 238µs ± 5% -98.41% (p=0.000 n=10+10) name old time/op new time/op delta XBenchGarbage-12 2.30ms ± 3% 2.29ms ± 1% -0.43% (p=0.049 n=17+18) name old time/op new time/op delta BinaryTree17-12 2.57s ± 3% 2.59s ± 2% ~ (p=0.141 n=19+20) Fannkuch11-12 2.09s ± 0% 2.10s ± 1% +0.53% (p=0.000 n=19+19) FmtFprintfEmpty-12 45.3ns ± 3% 45.2ns ± 2% ~ (p=0.845 n=20+20) FmtFprintfString-12 129ns ± 0% 127ns ± 0% -1.55% (p=0.000 n=16+16) FmtFprintfInt-12 123ns ± 0% 119ns ± 1% -3.24% (p=0.000 n=19+19) FmtFprintfIntInt-12 195ns ± 1% 189ns ± 1% -3.11% (p=0.000 n=17+17) FmtFprintfPrefixedInt-12 193ns ± 1% 187ns ± 1% -3.06% (p=0.000 n=19+19) FmtFprintfFloat-12 254ns ± 0% 255ns ± 1% +0.35% (p=0.001 n=14+17) FmtManyArgs-12 781ns ± 0% 770ns ± 0% -1.48% (p=0.000 n=16+19) GobDecode-12 7.00ms ± 1% 6.98ms ± 1% ~ (p=0.563 n=19+19) GobEncode-12 5.91ms ± 1% 5.92ms ± 0% ~ (p=0.118 n=19+18) Gzip-12 219ms ± 1% 215ms ± 1% -1.81% (p=0.000 n=18+18) Gunzip-12 37.2ms ± 0% 37.4ms ± 0% +0.45% (p=0.000 n=17+19) HTTPClientServer-12 76.9µs ± 3% 77.5µs ± 2% +0.81% (p=0.030 n=20+19) JSONEncode-12 15.0ms ± 0% 14.8ms ± 1% -0.88% (p=0.001 n=15+19) JSONDecode-12 50.6ms ± 0% 53.2ms ± 2% +5.07% (p=0.000 n=17+19) Mandelbrot200-12 4.05ms ± 0% 4.05ms ± 1% ~ (p=0.581 n=16+17) GoParse-12 3.34ms ± 1% 3.30ms ± 1% -1.21% (p=0.000 n=15+20) RegexpMatchEasy0_32-12 69.6ns ± 1% 69.8ns ± 2% ~ (p=0.566 n=19+19) RegexpMatchEasy0_1K-12 238ns ± 1% 236ns ± 0% -0.91% (p=0.000 n=17+13) RegexpMatchEasy1_32-12 69.8ns ± 1% 70.0ns ± 1% +0.23% (p=0.026 n=17+16) RegexpMatchEasy1_1K-12 371ns ± 1% 363ns ± 1% -2.07% (p=0.000 n=19+19) RegexpMatchMedium_32-12 107ns ± 2% 106ns ± 1% -0.51% (p=0.031 n=18+20) RegexpMatchMedium_1K-12 33.0µs ± 0% 32.9µs ± 0% -0.30% (p=0.004 n=16+16) RegexpMatchHard_32-12 1.70µs ± 0% 1.70µs ± 0% +0.45% (p=0.000 n=16+17) RegexpMatchHard_1K-12 51.1µs ± 2% 51.4µs ± 1% +0.53% (p=0.000 n=17+19) Revcomp-12 378ms ± 1% 385ms ± 1% +1.92% (p=0.000 n=19+18) Template-12 64.3ms ± 2% 65.0ms ± 2% +1.09% (p=0.001 n=19+19) TimeParse-12 315ns ± 1% 317ns ± 2% ~ (p=0.108 n=18+20) TimeFormat-12 360ns ± 1% 337ns ± 0% -6.30% (p=0.000 n=18+13) [Geo mean] 51.8µs 51.6µs -0.48% Change-Id: Icf8994671476840e3998236e15407a505d4c760c Reviewed-on: https://go-review.googlesource.com/20700Reviewed-by:Rick Hudson <rlh@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>
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