vtime: Use consistent clocks among nohz accounting

While computing the cputime delta of dynticks CPUs,
we are mixing up clocks of differents natures:

* local_clock() which takes care of unstable clock
sources and fix these if needed.

* sched_clock() which is the weaker version of
local_clock(). It doesn't compute any fixup in case
of unstable source.

If the clock source is stable, those two clocks are the
same and we can safely compute the difference against
two random points.

Otherwise it results in random deltas as sched_clock()
can randomly drift away, back or forward, from local_clock().

As a consequence, some strange behaviour with unstable tsc
has been observed such as non progressing constant zero cputime.
(The 'top' command showing no load).

Fix this by only using local_clock(), or its irq safe/remote
equivalent, in vtime code.

Reported-by: Mike Galbraith <efault@gmx.de>
Suggested-by: Mike Galbraith <efault@gmx.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 4 insertions, 4 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 58453b8272fd..e1a27f918723 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4745,7 +4745,7 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
          */
         idle->sched_class = &idle_sched_class;
         ftrace_graph_init_idle_task(idle, cpu);
-        vtime_init_idle(idle);
+        vtime_init_idle(idle, cpu);
 #if defined(CONFIG_SMP)
         sprintf(idle->comm, "%s/%d", INIT_TASK_COMM, cpu);
 #endif
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index cc2dc3eea8a3..b5ccba22603b 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -747,17 +747,17 @@ void arch_vtime_task_switch(struct task_struct *prev)
 
         write_seqlock(&current->vtime_seqlock);
         current->vtime_snap_whence = VTIME_SYS;
-        current->vtime_snap = sched_clock();
+        current->vtime_snap = sched_clock_cpu(smp_processor_id());
         write_sequnlock(&current->vtime_seqlock);
 }
 
-void vtime_init_idle(struct task_struct *t)
+void vtime_init_idle(struct task_struct *t, int cpu)
 {
         unsigned long flags;
 
         write_seqlock_irqsave(&t->vtime_seqlock, flags);
         t->vtime_snap_whence = VTIME_SYS;
-        t->vtime_snap = sched_clock();
+        t->vtime_snap = sched_clock_cpu(cpu);
         write_sequnlock_irqrestore(&t->vtime_seqlock, flags);
 }