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 b94f3541f67be00802c28f22fa5b56f9e8c91e55..a5b9a83065fae93e621eec1d2718c0a71b3803eb 100644 (file)
--- 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 6cc1fd5d5072b69638c562d7e01697d4c9870684..f91bc8141dc346c018a1ab6670ce8a6f1e9ab10f 100644 (file)
--- 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.
  *