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runtime: simplify TestSelectStackAdjust

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Currently this test attempts to trigger a concurrent GC in a very
indirect way, but the way it does so is extremely error-prone. This test
is virtually always prone to flaking based on test order. For example if
the test that executed immediately before this one made a big heap but
didn't clean it up, then this test could easily fail to trigger a GC.
I was able to prove this with a small reproducer.

This roundabout way of triggering a GC is also way overkill for this
test. It just wants to get goroutines in a select and shrink their
stacks. Every GC will schedule a stack for shrinking if it can.

Replace all the complicated machinery with a single runtime.GC call.
I've confirmed that the test consistently triggers a stack shrink,
noting that both shrinkstack's copystack call is made and that
syncadjustsudogs (the relevant function that's being indirectly tested)
are both called.

Fixes #44610.

Change-Id: Ib1c091e0d1475bf6c596f56dc9b85eaea366fc73
Reviewed-on: https://go-review.googlesource.com/c/go/+/313109
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
This commit is contained in:
Michael Anthony Knyszek 2021-04-23 18:11:52 +00:00 committed by Michael Knyszek
parent b8fed48a9a
commit e7db792fc5
1 changed files with 2 additions and 16 deletions
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18
src/runtime/chan_test.go
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@ -708,8 +708,6 @@ func TestSelectDuplicateChannel(t *testing.T) {
c <- 8 // wake up B. This operation used to fail because c.recvq was corrupted (it tries to wake up an already running G instead of B)
}
var selectSink interface{}
func TestSelectStackAdjust(t *testing.T) {
// Test that channel receive slots that contain local stack
// pointers are adjusted correctly by stack shrinking.
@ -766,20 +764,8 @@ func TestSelectStackAdjust(t *testing.T) {
<-ready2
time.Sleep(10 * time.Millisecond)
// Force concurrent GC a few times.
var before, after runtime.MemStats
runtime.ReadMemStats(&before)
for i := 0; i < 100; i++ {
selectSink = new([1 << 20]byte)
runtime.ReadMemStats(&after)
if after.NumGC-before.NumGC >= 2 {
goto done
}
runtime.Gosched()
}
t.Fatal("failed to trigger concurrent GC")
done:
selectSink = nil
// Force concurrent GC to shrink the stacks.
runtime.GC()
// Wake selects.
close(d)