runtime: push down systemstack requirement for tracer where possible
Currently lots of functions require systemstack because the trace buffer
might get flushed, but that will already switch to the systemstack for
the most critical bits (grabbing trace.lock). That means a lot of this
code is non-preemptible when it doesn't need to be. We've seen this
cause problems at scale, when dumping very large numbers of stacks at
once, for example.
Change-Id: I88340091a3c43f0513b5601ef5199c946aa56ed7
Reviewed-on: https://go-review.googlesource.com/c/go/+/572095
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>