Remove ld in example toolchain and use gcc
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Summary: The stack around D42099161 introduced the ability to dynamically set the log level via a debug-command. This requires using `tracing_subscriber::reload::Layer`. The implementation of that machinery is backed by a `RwLock` [here](https://fburl.com/code/4xv0ihpn). This `RwLock` is read locked once on every single `#[instrumentation]` call to check whether the instrumentation is active or not. On top of that, something in our fbsource third-party config enables the `parking_lot` feature of `tracing_subscriber`. This means that this is a `parking_lot::RwLock`, not a `std::sync::RwLock`, which is bad because it's well known that parking lot has problems with excessive spinning. What all that adds up to is that when you put `#[instrument]` on a super hot function in dice, you get dozens of threads that are all simultaneously doing this: ``` thread #23, name = 'buck2-rt', stop reason = signal SIGSTOP frame #0: 0x000000000c94a1fa buck2`<parking_lot::raw_rwlock::RawRwLock>::lock_shared_slow + 122 frame #1: 0x0000000007a80b54 buck2`<tracing_subscriber::layer::layered::Layered<tracing_subscriber::reload::Layer<tracing_subscriber::filter::layer_filters::Filtered<tracing_subscriber::fmt::fmt_layer::Layer<tracing_subscriber::registry::sharded::Registry, tracing_subscriber::fmt::format::DefaultFields, tracing_subscriber::fmt::format::Format, std::io::stdio::stderr>, tracing_subscriber::filter::env::EnvFilter, tracing_subscriber::registry::sharded::Registry>, tracing_subscriber::registry::sharded::Registry>, tracing_subscriber::registry::sharded::Registry> as tracing_core::subscriber::Subscriber>::enabled + 292 frame #2: 0x000000000a172d77 buck2`<dice::legacy::incremental::dep_trackers::internals::Dep<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>> as dice::legacy::incremental::graph::dependencies::Dependency>::recompute::{closure#0} + 183 frame #3: 0x000000000a03606c buck2`<futures_util::stream::futures_unordered::FuturesUnordered<core::pin::Pin<alloc::boxed::Box<dyn core::future::future::Future<Output = core::result::Result<(alloc::boxed::Box<dyn dice::legacy::incremental::graph::dependencies::ComputedDependency>, alloc::sync::Arc<dyn dice::legacy::incremental::graph::GraphNodeDyn>), dice::api::error::DiceError>> + core::marker::Send>>> as futures_util::stream::stream::StreamExt>::poll_next_unpin + 444 frame #4: 0x0000000009fc4755 buck2`<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::compute_whether_dependencies_changed::{closure#0} (.llvm.4229350879637282184) + 1733 frame #5: 0x0000000009ef30d4 buck2`<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::compute_whether_versioned_dependencies_changed::{closure#0}::{closure#0} + 228 frame #6: 0x0000000009f194ec buck2`<buck2_futures::cancellable_future::CancellableFuture<<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::new_dice_task::{closure#0}> as core::future::future::Future>::poll (.llvm.11181184606289051452) + 3500 frame #7: 0x0000000009f04bbf buck2`<futures_util::future::future::map::Map<buck2_futures::cancellable_future::CancellableFuture<<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::new_dice_task::{closure#0}>, buck2_futures::spawn::spawn_inner<<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::new_dice_task::{closure#0}, dice::api::user_data::UserComputationData, futures_util::future::ready::Ready<()>>::{closure#0}> as core::future::future::Future>::poll + 31 frame #8: 0x0000000009ff0339 buck2`<futures_util::future::future::flatten::Flatten<futures_util::future::future::Map<futures_util::future::ready::Ready<()>, buck2_futures::spawn::spawn_inner<<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::new_dice_task::{closure#0}, dice::api::user_data::UserComputationData, futures_util::future::ready::Ready<()>>::{closure#1}>, <futures_util::future::future::Map<futures_util::future::ready::Ready<()>, buck2_futures::spawn::spawn_inner<<dice::legacy::incremental::IncrementalEngine<dice::legacy::key::StoragePropertiesForKey<buck2_build_api::actions::calculation::BuildKey>>>::new_dice_task::{closure#0}, dice::api::user_data::UserComputationData, futures_util::future::ready::Ready<()>>::{closure#1}> as core::future::future::Future>::Output> as core::future::future::Future>::poll + 201 frame #9: 0x00000000093f9f22 buck2`<tokio::task::task_local::TaskLocalFuture<buck2_events::dispatch::EventDispatcher, core::pin::Pin<alloc::boxed::Box<dyn core::future::future::Future<Output = alloc::boxed::Box<dyn core::any::Any + core::marker::Send>> + core::marker::Send>>> as core::future::future::Future>::poll + 130 frame #10: 0x000000000939fdcb buck2`<tracing::instrument::Instrumented<<buck2_build_api::spawner::BuckSpawner as buck2_futures::spawner::Spawner<dice::api::user_data::UserComputationData>>::spawn::{closure#0}> as core::future::future::Future>::poll + 139 frame #11: 0x00000000091ca5a9 buck2`<tokio::runtime::task::core::Core<tracing::instrument::Instrumented<<buck2_build_api::spawner::BuckSpawner as buck2_futures::spawner::Spawner<dice::api::user_data::UserComputationData>>::spawn::{closure#0}>, alloc::sync::Arc<tokio::runtime::scheduler::multi_thread::handle::Handle>>>::poll + 169 frame #12: 0x00000000091c1b22 buck2`<tokio::runtime::task::harness::Harness<tracing::instrument::Instrumented<<buck2_build_api::spawner::BuckSpawner as buck2_futures::spawner::Spawner<dice::api::user_data::UserComputationData>>::spawn::{closure#0}>, alloc::sync::Arc<tokio::runtime::scheduler::multi_thread::handle::Handle>>>::poll + 146 frame #13: 0x000000000c920f6f buck2`<tokio::runtime::scheduler::multi_thread::worker::Context>::run_task + 895 frame #14: 0x000000000c91f847 buck2`<tokio::runtime::scheduler::multi_thread::worker::Context>::run + 2103 frame #15: 0x000000000c932264 buck2`<tokio::runtime::context::scoped::Scoped<tokio::runtime::scheduler::Context>>::set::<tokio::runtime::scheduler::multi_thread::worker::run::{closure#0}::{closure#0}, ()> + 52 frame #16: 0x000000000c932169 buck2`tokio::runtime::context::runtime::enter_runtime::<tokio::runtime::scheduler::multi_thread::worker::run::{closure#0}, ()> + 441 frame #17: 0x000000000c91efa6 buck2`tokio::runtime::scheduler::multi_thread::worker::run + 70 frame #18: 0x000000000c906a50 buck2`<tracing::instrument::Instrumented<tokio::runtime::blocking::task::BlockingTask<<tokio::runtime::scheduler::multi_thread::worker::Launch>::launch::{closure#0}>> as core::future::future::Future>::poll + 160 frame #19: 0x000000000c8f8af9 buck2`<tokio::loom::std::unsafe_cell::UnsafeCell<tokio::runtime::task::core::Stage<tracing::instrument::Instrumented<tokio::runtime::blocking::task::BlockingTask<<tokio::runtime::scheduler::multi_thread::worker::Launch>::launch::{closure#0}>>>>>::with_mut::<core::task::poll::Poll<()>, <tokio::runtime::task::core::Core<tracing::instrument::Instrumented<tokio::runtime::blocking::task::BlockingTask<<tokio::runtime::scheduler::multi_thread::worker::Launch>::launch::{closure#0}>>, tokio::runtime::blocking::schedule::BlockingSchedule>>::poll::{closure#0}> + 153 frame #20: 0x000000000c90166b buck2`<tokio::runtime::task::core::Core<tracing::instrument::Instrumented<tokio::runtime::blocking::task::BlockingTask<<tokio::runtime::scheduler::multi_thread::worker::Launch>::launch::{closure#0}>>, tokio::runtime::blocking::schedule::BlockingSchedule>>::poll + 43 frame #21: 0x000000000c90d9b8 buck2`<tokio::runtime::task::harness::Harness<tracing::instrument::Instrumented<tokio::runtime::blocking::task::BlockingTask<<tokio::runtime::scheduler::multi_thread::worker::Launch>::launch::{closure#0}>>, tokio::runtime::blocking::schedule::BlockingSchedule>>::poll + 152 frame #22: 0x000000000c90b848 buck2`<tokio::runtime::blocking::pool::Inner>::run + 216 frame #23: 0x000000000c9002ab buck2`std::sys_common::backtrace::__rust_begin_short_backtrace::<<tokio::runtime::blocking::pool::Spawner>::spawn_thread::{closure#0}, ()> + 187 frame #24: 0x000000000c90042e buck2`<<std::thread::Builder>::spawn_unchecked_<<tokio::runtime::blocking::pool::Spawner>::spawn_thread::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} + 158 frame #25: 0x000000000757e4b5 buck2`std::sys::unix::thread::Thread::new::thread_start::h618b0b8e7bda0b2b + 37 frame #26: 0x00007f2ba1e9abaf ``` This hit an open source user super hard over in #555 . This diff approaches this issue by putting all the `#[instrument]`s in dice behind `#[cfg(debug_assertions)]`. This allows them to still be used in debug mode, but keeps them out of any hot paths. I do not have numbers to show that most of these matter. The only one I know matters for sure is `recompute`. Alternatives are to either try and tailor this to the callsites we know are hot (that sounds error prone) or to revert the stack that inadvertently introduced the RwLock. Even if we did that though, it's probably still safer to keep `#[instrument]` out of super hot paths like this. Reviewed By: stepancheg Differential Revision: D53744041 fbshipit-source-id: 85bce9af2fec8ad1d50adc241d3b8e2bfc5cec87
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This diff favours gcc over ld, allowing for autodiscovery of link paths.