MemProf.pretty_log_allocations hangs indefinitely when used with -Dpreview_mt

[foxxx0]

Binaries built with -Dpreview_mt cause MemProf.pretty_log_allocations to just hang and never complete.

Minimal example for reproduction:

require "perf_tools/mem_prof"

class Foo; end

_ = Array.new(100_000) { Foo.new }

PerfTools::MemProf.pretty_log_allocations(STDOUT)

When built without enabling preview_mt this runs just fine, for both normal (unoptimized) builds as well as --release builds.

However, as soon as -Dpreview_mt is flipped on, this will no longer complete and just get stuck.

I have not the slightest clue as to why that may be, maybe I'm just "holding it wrong"?

[HertzDevil]

Most likely we will have to stop the world

[ysbaddaden]

PerfTools::MemProf is thread unsafe.

The log methods shall stop the world, but that won't be enough:

• We must protect the global class variables (alloc_infos, ...). Maybe use Sync::Map instead of Hash? Or maybe thread locals but then we must consolidate before each log.

• We can't just stop the GC and skip track/untrack while @@running is false because another thread can be allocating normally. Maybe it could be a thread local?

Note: Sync::Map would need unsafe iteration methods because a thread could get stopped while holding a write lock.