sched: Remove rq->clock coupling from set_task_cpu()

set_task_cpu() should be rq invariant and only touch task state, it
currently fails to do so, which opens up a few races, since not all
callers hold both rq->locks.

Remove the relyance on rq->clock, as any site calling set_task_cpu()
should also do a remote clock update, which should ensure the observed
time between these two cpus is monotonic, as per
kernel/sched_clock.c:sched_clock_remote().

Therefore we can simply remove the clock_offset bits and be happy.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -2060,23 +2060,12 @@ task_hot(struct task_struct *p, u64 now, struct sched_domain *sd)
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 {
 	int old_cpu = task_cpu(p);
-	struct rq *old_rq = cpu_rq(old_cpu), *new_rq = cpu_rq(new_cpu);
+	struct rq *old_rq = cpu_rq(old_cpu);
 	struct cfs_rq *old_cfsrq = task_cfs_rq(p),
 		      *new_cfsrq = cpu_cfs_rq(old_cfsrq, new_cpu);
-	u64 clock_offset;
-
-	clock_offset = old_rq->clock - new_rq->clock;
 
 	trace_sched_migrate_task(p, new_cpu);
 
-#ifdef CONFIG_SCHEDSTATS
-	if (p->se.wait_start)
-		p->se.wait_start -= clock_offset;
-	if (p->se.sleep_start)
-		p->se.sleep_start -= clock_offset;
-	if (p->se.block_start)
-		p->se.block_start -= clock_offset;
-#endif
 	if (old_cpu != new_cpu) {
 		p->se.nr_migrations++;
 #ifdef CONFIG_SCHEDSTATS