runtime/pprof: TestLabelSystemstack failures with "no samples in expected functions"

[bcmills]

--- FAIL: TestLabelSystemstack (7.24s)
    pprof_test.go:526: total 36 CPU profile samples collected:
        1: 0x410e23 (runtime.(*mspan).refillAllocCache:153) 0x410ff1 (runtime.(*mspan).nextFreeIndex:211) 0x40b215 (runtime.(*mcache).nextFree:879) 0x40b6ba (runtime.mallocgc:1060) 0x409565 (runtime.convT32:372) 0x547ae8 (runtime/pprof.labelHog:1476) 0x54773b (runtime/pprof.TestLabelSystemstack.func1.1.1:1437) labels: map[key:[value]]
[…]
    pprof_test.go:597: runtime.systemstack;key=value: 0
    pprof_test.go:600: no samples in expected functions
    pprof_test.go:611: runtime.systemstack;key=value has 0 samples out of 0, want at least 1, ideally 0
    pprof_test.go:462: retrying with 3.2s duration
    pprof_test.go:526: total 1279 CPU profile samples collected:
        8: 0x46c11f (sync.(*Pool).Put:103) 0x4b4222 (fmt.(*pp).free:161) 0x4b449d (fmt.Fprintf:206) 0x547b3c (runtime/pprof.labelHog:1476) 0x54773b (runtime/pprof.TestLabelSystemstack.func1.1.1:1437) labels: map[key:[value]]
        
        1: 0x4610f0 (sync.runtime_procUnpin:6265) 0x46c175 (sync.(*Pool).Put:110) 0x4b4222 (fmt.(*pp).free:161) 0x4b449d (fmt.Fprintf:206) 0x547b3c (runtime/pprof.labelHog:1476) 0x54773b (runtime/pprof.TestLabelSystemstack.func1.1.1:1437) labels: map[key:[value]]
[…]
    pprof_test.go:597: runtime.systemstack;key=value: 0
    pprof_test.go:600: no samples in expected functions
    pprof_test.go:611: runtime.systemstack;key=value has 0 samples out of 0, want at least 1, ideally 0

greplogs --dashboard -md -l -e 'FAIL: TestLabelSystemstack .*\n(?: .+\n)* .*no samples in expected functions' --since=2021-12-07

2021-12-08T15:30:52-9fe77de/windows-386-2008

(attn @felixge @prattmic)

[bcmills]

(Release blocker for Go 1.18 because this test is new as of 1.18.)

[prattmic]

I think this is just a case of getting unlucky and not getting the sample we want in time. fmt.Fprintf way not be the best sink here, I'll look around for a better one.

[gopherbot]

Change https://golang.org/cl/370375 mentions this issue: runtime/pprof: increase systemstack calls in TestLabelSystemstack