runtime/cgo: store M for C-created thread in pthread key

In a C thread, it's necessary to acquire an extra M by using needm while invoking a Go function from C. But, needm and dropm are heavy costs due to the signal-related syscalls.
So, we change to not dropm while returning back to C, which means binding the extra M to the C thread until it exits, to avoid needm and dropm on each C to Go call.
Instead, we only dropm while the C thread exits, so the extra M won't leak.

When invoking a Go function from C:
Allocate a pthread variable using pthread_key_create, only once per shared object, and register a thread-exit-time destructor.
And store the g0 of the current m into the thread-specified value of the pthread key,  only once per C thread, so that the destructor will put the extra M back onto the extra M list while the C thread exits.

When returning back to C:
Skip dropm in cgocallback, when the pthread variable has been created, so that the extra M will be reused the next time invoke a Go function from C.

This is purely a performance optimization. The old version, in which needm & dropm happen on each cgo call, is still correct too, and we have to keep the old version on systems with cgo but without pthreads, like Windows.

This optimization is significant, and the specific value depends on the OS system and CPU, but in general, it can be considered as 10x faster, for a simple Go function call from a C thread.

For the newly added BenchmarkCGoInCThread, some benchmark results:
1. it's 28x faster, from 3395 ns/op to 121 ns/op, in darwin OS & Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
2. it's 6.5x faster, from 1495 ns/op to 230 ns/op, in Linux OS & Intel(R) Xeon(R) CPU E5-2630 0 @ 2.30GHz

Fixes #51676

Change-Id: I380702fe2f9b6b401b2d6f04b0aba990f4b9ee6c
GitHub-Last-Rev: 93dc64ad98e5583372e41f65ee4b7ab78b5aff51
GitHub-Pull-Request: golang/go#51679
Reviewed-on: https://go-review.googlesource.com/c/go/+/392854
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: thepudds <thepudds1460@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/src/runtime/cgo/callbacks.go b/src/runtime/cgo/callbacks.go
index e7c8ef3e07..792dd7d086 100644
--- a/src/runtime/cgo/callbacks.go
+++ b/src/runtime/cgo/callbacks.go
@@ -71,6 +71,42 @@ var _cgo_thread_start = &x_cgo_thread_start
 var x_cgo_sys_thread_create byte
 var _cgo_sys_thread_create = &x_cgo_sys_thread_create
 
+// Indicates whether a dummy thread key has been created or not.
+//
+// When calling go exported function from C, we register a destructor
+// callback, for a dummy thread key, by using pthread_key_create.
+
+//go:cgo_import_static x_cgo_pthread_key_created
+//go:linkname x_cgo_pthread_key_created x_cgo_pthread_key_created
+//go:linkname _cgo_pthread_key_created _cgo_pthread_key_created
+var x_cgo_pthread_key_created byte
+var _cgo_pthread_key_created = &x_cgo_pthread_key_created
+
+// Export crosscall2 to a c function pointer variable.
+// Used to dropm in pthread key destructor, while C thread is exiting.
+
+//go:cgo_import_static x_crosscall2_ptr
+//go:linkname x_crosscall2_ptr x_crosscall2_ptr
+//go:linkname _crosscall2_ptr _crosscall2_ptr
+var x_crosscall2_ptr byte
+var _crosscall2_ptr = &x_crosscall2_ptr
+
+// Set the x_crosscall2_ptr C function pointer variable point to crosscall2.
+// It's for the runtime package to call at init time.
+func set_crosscall2()
+
+//go:linkname _set_crosscall2 runtime.set_crosscall2
+var _set_crosscall2 = set_crosscall2
+
+// Store the g into the thread-specific value.
+// So that pthread_key_destructor will dropm when the thread is exiting.
+
+//go:cgo_import_static x_cgo_bindm
+//go:linkname x_cgo_bindm x_cgo_bindm
+//go:linkname _cgo_bindm _cgo_bindm
+var x_cgo_bindm byte
+var _cgo_bindm = &x_cgo_bindm
+
 // Notifies that the runtime has been initialized.
 //
 // We currently block at every CGO entry point (via _cgo_wait_runtime_init_done)