From: Markus Metzger <markus.t.metzger@intel.com>
Date: Fri, 3 Apr 2009 14:43:34 +0000 (+0200)
Subject: sched, hw-branch-tracer: add wait_task_context_switch() function to sched.h
X-Git-Tag: v2.6.31-rc1~401^2~62^2~25
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=a26b89f05d194413c7238e0bea071054f6b5d3c8;p=karo-tx-linux.git

sched, hw-branch-tracer: add wait_task_context_switch() function to sched.h

Add a function to wait until some other task has been
switched out at least once.

This differs from wait_task_inactive() subtly, in that the
latter will wait until the task has left the CPU.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: markus.t.metzger@gmail.com
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: oleg@redhat.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144549.794157000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b94f3541f67b..a5b9a83065fa 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1993,8 +1993,10 @@ extern void set_task_comm(struct task_struct *tsk, char *from);
 extern char *get_task_comm(char *to, struct task_struct *tsk);
 
 #ifdef CONFIG_SMP
+extern void wait_task_context_switch(struct task_struct *p);
 extern unsigned long wait_task_inactive(struct task_struct *, long match_state);
 #else
+static inline void wait_task_context_switch(struct task_struct *p) {}
 static inline unsigned long wait_task_inactive(struct task_struct *p,
 					       long match_state)
 {
diff --git a/kernel/sched.c b/kernel/sched.c
index 6cc1fd5d5072..f91bc8141dc3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2002,6 +2002,49 @@ migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
 	return 1;
 }
 
+/*
+ * wait_task_context_switch -	wait for a thread to complete at least one
+ *				context switch.
+ *
+ * @p must not be current.
+ */
+void wait_task_context_switch(struct task_struct *p)
+{
+	unsigned long nvcsw, nivcsw, flags;
+	int running;
+	struct rq *rq;
+
+	nvcsw	= p->nvcsw;
+	nivcsw	= p->nivcsw;
+	for (;;) {
+		/*
+		 * The runqueue is assigned before the actual context
+		 * switch. We need to take the runqueue lock.
+		 *
+		 * We could check initially without the lock but it is
+		 * very likely that we need to take the lock in every
+		 * iteration.
+		 */
+		rq = task_rq_lock(p, &flags);
+		running = task_running(rq, p);
+		task_rq_unlock(rq, &flags);
+
+		if (likely(!running))
+			break;
+		/*
+		 * The switch count is incremented before the actual
+		 * context switch. We thus wait for two switches to be
+		 * sure at least one completed.
+		 */
+		if ((p->nvcsw - nvcsw) > 1)
+			break;
+		if ((p->nivcsw - nivcsw) > 1)
+			break;
+
+		cpu_relax();
+	}
+}
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *