ircd::ctx: Add state for counting context switches.

include/ircd/ctx/ctx.h

@@ -53,6 +53,7 @@ namespace ircd::ctx
 	const size_t &stack_max(const ctx &);        // Returns stack size allocated for ctx
 	const size_t &stack_at(const ctx &);         // Stack at last sleep (also see this_ctx.h)
 	const int64_t &notes(const ctx &);           // Peeks at internal semaphore count
+	const uint64_t &yields(const ctx &);         // Context switching counter
 	const ulong &cycles(const ctx &);            // Accumulated tsc (not counting cur slice)
 	bool interruption(const ctx &);              // Context was marked for interruption
 	bool termination(const ctx &);               // Context was marked for termination

ircd/ctx.cc

@@ -381,6 +381,13 @@ ircd::ctx::cycles(const ctx &ctx)
 	return ctx.cycles;
 }
 
+/// Returns the yield count for `ctx`
+const uint64_t &
+ircd::ctx::yields(const ctx &ctx)
+{
+	return ctx.yields;
+}
+
 /// Returns the notification count for `ctx`
 const int64_t &
 ircd::ctx::notes(const ctx &ctx)
@@ -636,6 +643,7 @@ ircd::ctx::continuation::continuation(ctx *const &self)
 	assert(!std::current_exception());
 	//assert(!std::uncaught_exceptions());
 
+	self->yields++;
 	self->cont = this;
 	ircd::ctx::current = nullptr;
 }

ircd/ctx.h

@@ -28,6 +28,7 @@ struct ircd::ctx::ctx
 	size_t stack_at {0};                         // Updated for profiling at sleep
 	int64_t notes {0};                           // norm: 0 = asleep; 1 = awake; inc by others; dec by self
 	ulong cycles {0};                            // monotonic counter (rdtsc)
+	uint64_t yields {0};                         // monotonic counter
 	continuation *cont {nullptr};                // valid when asleep; invalid when awake
 	ctx *adjoindre {nullptr};                    // context waiting for this to join()
 	list::node node;                             // node for ctx::list