[perf] Reduce contention in ptrace.threadPool.lookupOrCreate().

lookupOrCreate is called from subprocess.switchToApp() and subprocess.syscall().
lookupOrCreate() looks for a thread already created for the current TID. If a
thread exists (common case), it returns immediately. Otherwise it creates a new
one.

This change switches to using a sync.RWMutex. The initial thread existence
lookup is now done only with the read lock. So multiple successful lookups can
occur concurrently. Only when a new thread is created will it acquire the lock
for writing and update the map (which is not the common case).

Discovered in mutex profiles from the various ptrace benchmarks.
Example: https://gvisor.dev/profile/gvisor-buildkite/fd14bfad-b30f-44dc-859b-80ebac50beb4/843827db-da50-4dc9-a2ea-ecf734dde2d5/tmp/profile/ptrace/BenchmarkFio/operation.write/blockSize.4K/filesystem.tmpfs/benchmarks/fio/mutex.pprof/flamegraph
PiperOrigin-RevId: 365612094

0 files changed, 0 insertions, 0 deletions