runtime: eliminate possible stack movements in ReadMetricsSlow

Currently it's possible (and even probable, with mayMoreStackMove mode)
for a stack allocation to occur between readmemstats_m and readMetrics
in ReadMetricsSlow. This can cause tests to fail by producing metrics
that are inconsistent between the two sources.

Fix this by breaking out the critical section of readMetrics and calling
that from ReadMetricsSlow on the systemstack. Our main constraint in
calling readMetrics on the system stack is the fact that we can't
acquire the metrics semaphore from the system stack. But if we break out
the critical section, then we can acquire that semaphore before we go on
the system stack.

While we're here, add another readMetrics call before readmemstats_m.
Since we're being paranoid about ways that metrics could get skewed
between the two calls, let's eliminate all uncertainty. It's possible
for readMetrics to allocate new memory, for example for histograms, and
fail while it's reading metrics. I believe we're just getting lucky
today with the order in which the metrics are produced. Another call to
readMetrics will preallocate this data in the samples slice. One nice
thing about this second read is that now we effectively have a way to
check if readMetrics really will allocate if called a second time on the
same samples slice.

Fixes #60607.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Change-Id: If6ce666530903239ef9f02dbbc3f1cb6be71e425
Reviewed-on: https://go-review.googlesource.com/c/go/+/539117
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

0 files changed, 0 insertions, 0 deletions