runtime: get traceback from VDSO code

Currently if a profiling signal arrives while executing within a VDSO
the profiler will report _ExternalCode, which is needlessly confusing
for a pure Go program. Change the VDSO calling code to record the
caller's PC/SP, so that we can do a traceback from that point. If that
fails for some reason, report _VDSO rather than _ExternalCode, which
should at least point in the right direction.

This adds some instructions to the code that calls the VDSO, but the
slowdown is reasonably negligible:

name                                  old time/op  new time/op  delta
ClockVDSOAndFallbackPaths/vDSO-8      40.5ns ± 2%  41.3ns ± 1%  +1.85%  (p=0.002 n=10+10)
ClockVDSOAndFallbackPaths/Fallback-8  41.9ns ± 1%  43.5ns ± 1%  +3.84%  (p=0.000 n=9+9)
TimeNow-8                             41.5ns ± 3%  41.5ns ± 2%    ~     (p=0.723 n=10+10)

Fixes #24142

Change-Id: Iacd935db3c4c782150b3809aaa675a71799b1c9c
Reviewed-on: https://go-review.googlesource.com/97315
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/runtime/vdso_linux.go b/src/runtime/vdso_linux.go
index 6f4c5bb83e..7939bb54be 100644
--- a/src/runtime/vdso_linux.go
+++ b/src/runtime/vdso_linux.go
@@ -279,3 +279,14 @@ func vdsoauxv(tag, val uintptr) {
 		vdsoParseSymbols(info1, vdsoFindVersion(info1, &linux26))
 	}
 }
+
+// vdsoMarker returns whether PC is on the VDSO page.
+func inVDSOPage(pc uintptr) bool {
+	for _, k := range vdsoSymbolKeys {
+		if *k.ptr != 0 {
+			page := *k.ptr &^ (physPageSize - 1)
+			return pc >= page && pc < page+physPageSize
+		}
+	}
+	return false
+}