[clang-tools-extra] r334495 - [clangd] Trace time the operations wait on Semaphore.

[Ilya Biryukov via cfe-commits]

Author: ibiryukov
Date: Tue Jun 12 04:56:21 2018
New Revision: 334495

URL: http://llvm.org/viewvc/llvm-project?rev=334495&view=rev
Log:
[clangd] Trace time the operations wait on Semaphore.

The Semaphore is currently used to limit the number of concurrently
running tasks. Tracing the wait times will allow to find out how much
time is wasted waiting on other operations to complete.

Modified:
    clang-tools-extra/trunk/clangd/Threading.cpp

Modified: clang-tools-extra/trunk/clangd/Threading.cpp
URL: http://llvm.org/viewvc/llvm-project/clang-tools-extra/trunk/clangd/Threading.cpp?rev=334495&r1=334494&r2=334495&view=diff
==============================================================================
--- clang-tools-extra/trunk/clangd/Threading.cpp (original)
+++ clang-tools-extra/trunk/clangd/Threading.cpp Tue Jun 12 04:56:21 2018
@@ -1,4 +1,5 @@
 #include "Threading.h"
+#include "Trace.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Threading.h"
@@ -23,9 +24,14 @@ void Notification::wait() const {
 Semaphore::Semaphore(std::size_t MaxLocks) : FreeSlots(MaxLocks) {}
 
 void Semaphore::lock() {
-  std::unique_lock<std::mutex> Lock(Mutex);
-  SlotsChanged.wait(Lock, [&]() { return FreeSlots > 0; });
-  --FreeSlots;
+  trace::Span Span("WaitForFreeSemaphoreSlot");
+  // trace::Span can also acquire locks in ctor and dtor, we make sure it
+  // happens when Semaphore's own lock is not held.
+  {
+    std::unique_lock<std::mutex> Lock(Mutex);
+    SlotsChanged.wait(Lock, [&]() { return FreeSlots > 0; });
+    --FreeSlots;
+  }
 }
 
 void Semaphore::unlock() {