runtime: avoid MADV_HUGEPAGE for heap memory

Currently the runtime marks all new memory as MADV_HUGEPAGE on Linux and
manages its hugepage eligibility status. Unfortunately, the default
THP behavior on most Linux distros is that MADV_HUGEPAGE blocks while
the kernel eagerly reclaims and compacts memory to allocate a hugepage.

This direct reclaim and compaction is unbounded, and may result in
significant application thread stalls. In really bad cases, this can
exceed 100s of ms or even seconds.

Really all we want is to undo MADV_NOHUGEPAGE marks and let the default
Linux paging behavior take over, but the only way to unmark a region as
MADV_NOHUGEPAGE is to also mark it MADV_HUGEPAGE.

The overall strategy of trying to keep hugepages for the heap unbroken
however is sound. So instead let's use the new shiny MADV_COLLAPSE if it
exists.

MADV_COLLAPSE makes a best-effort synchronous attempt at collapsing the
physical memory backing a memory region into a hugepage. We'll use
MADV_COLLAPSE where we would've used MADV_HUGEPAGE, and stop using
MADV_NOHUGEPAGE altogether.

Because MADV_COLLAPSE is synchronous, it's also important to not
re-collapse huge pages if the huge pages are likely part of some large
allocation. Although in many cases it's advantageous to back these
allocations with hugepages because they're contiguous, eagerly
collapsing every hugepage means having to page in at least part of the
large allocation.

However, because we won't use MADV_NOHUGEPAGE anymore, we'll no longer
handle the fact that khugepaged might come in and back some memory we
returned to the OS with a hugepage. I've come to the conclusion that
this is basically unavoidable without a new madvise flag and that it's
just not a good default. If this change lands, advice about Linux huge
page settings will be added to the GC guide.

Verified that this change doesn't regress Sweet, at least not on my
machine with:

/sys/kernel/mm/transparent_hugepage/enabled [always or madvise]
/sys/kernel/mm/transparent_hugepage/defrag [madvise]
/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none [0 or 511]

Unfortunately, this workaround means that we only get forced hugepages
on Linux 6.1+.

Fixes #61718.

Change-Id: I7f4a7ba397847de29f800a99f9cb66cb2720a533
Reviewed-on: https://go-review.googlesource.com/c/go/+/516795
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

src/runtime/export_test.go

@@ -1820,8 +1820,8 @@ func (s *ScavengeIndex) SetEmpty(ci ChunkIdx) {
 	s.i.setEmpty(chunkIdx(ci))
 }
 
-func (s *ScavengeIndex) SetNoHugePage(ci ChunkIdx) bool {
-	return s.i.setNoHugePage(chunkIdx(ci))
+func (s *ScavengeIndex) SetNoHugePage(ci ChunkIdx) {
+	s.i.setNoHugePage(chunkIdx(ci))
 }
 
 func CheckPackScavChunkData(gen uint32, inUse, lastInUse uint16, flags uint8) bool {