cputime: Use local_clock() for full dynticks cputime accounting

Running the full dynticks cputime accounting with preemptible
kernel debugging trigger the following warning:

	[    4.488303] BUG: using smp_processor_id() in preemptible [00000000] code: init/1
	[    4.490971] caller is native_sched_clock+0x22/0x80
	[    4.493663] Pid: 1, comm: init Not tainted 3.8.0+ #13
	[    4.496376] Call Trace:
	[    4.498996]  [<ffffffff813410eb>] debug_smp_processor_id+0xdb/0xf0
	[    4.501716]  [<ffffffff8101e642>] native_sched_clock+0x22/0x80
	[    4.504434]  [<ffffffff8101db99>] sched_clock+0x9/0x10
	[    4.507185]  [<ffffffff81096ccd>] fetch_task_cputime+0xad/0x120
	[    4.509916]  [<ffffffff81096dd5>] task_cputime+0x35/0x60
	[    4.512622]  [<ffffffff810f146e>] acct_update_integrals+0x1e/0x40
	[    4.515372]  [<ffffffff8117d2cf>] do_execve_common+0x4ff/0x5c0
	[    4.518117]  [<ffffffff8117cf14>] ? do_execve_common+0x144/0x5c0
	[    4.520844]  [<ffffffff81867a10>] ? rest_init+0x160/0x160
	[    4.523554]  [<ffffffff8117d457>] do_execve+0x37/0x40
	[    4.526276]  [<ffffffff810021a3>] run_init_process+0x23/0x30
	[    4.528953]  [<ffffffff81867aac>] kernel_init+0x9c/0xf0
	[    4.531608]  [<ffffffff8188356c>] ret_from_fork+0x7c/0xb0

We use sched_clock() to perform and fixup the cputime
accounting. However we are calling it with preemption enabled
from the read side, which trigger the bug above.

To fix this up, use local_clock() instead. It takes care of
preemption and also provide a more reliable clock source. This
is welcome for this kind of statistic that is widely relied on
in userspace.

Reported-by: Thomas Gleixner <[email protected]>
Reported-by: Ingo Molnar <[email protected]>
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Cc: Li Zhong <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Kevin Hilman <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/cputime.c

@@ -604,7 +604,7 @@ static unsigned long long vtime_delta(struct task_struct *tsk)
 {
 	unsigned long long clock;
 
-	clock = sched_clock();
+	clock = local_clock();
 	if (clock < tsk->vtime_snap)
 		return 0;