Commit 45eacc69 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Ingo Molnar
Browse files

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: default avatarMike Galbraith <efault@gmx.de>
Suggested-by: default avatarMike 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: default avatarFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 67dd331c

include/linux/vtime.h

+2 −2
Original line number Diff line number Diff line
@@ -34,7 +34,7 @@ static inline void vtime_user_exit(struct task_struct *tsk) 
}

extern void vtime_guest_enter(struct task_struct *tsk);

extern void vtime_guest_exit(struct task_struct *tsk);

extern void vtime_init_idle(struct task_struct *tsk);

extern void vtime_init_idle(struct task_struct *tsk, int cpu);

#else

static inline void vtime_account_irq_exit(struct task_struct *tsk)

{
@@ -45,7 +45,7 @@ static inline void vtime_user_enter(struct task_struct *tsk) { } 
static inline void vtime_user_exit(struct task_struct *tsk) { }

static inline void vtime_guest_enter(struct task_struct *tsk) { }

static inline void vtime_guest_exit(struct task_struct *tsk) { }

static inline void vtime_init_idle(struct task_struct *tsk) { }

static inline void vtime_init_idle(struct task_struct *tsk, int cpu) { }

#endif



#ifdef CONFIG_IRQ_TIME_ACCOUNTING

kernel/sched/core.c

+1 −1
Original line number Diff line number Diff line
@@ -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

kernel/sched/cputime.c

+3 −3
Original line number Diff line number Diff line
@@ -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);

}

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