Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Aug 23, 2015 at 07:40:32AM +0200, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > From: Vatika Harlalka <vatikaharlalka@gmail.com>
> > 
> > The problem addressed in this patch is about affining unpinned timers.
> > Adaptive or Full Dynticks CPUs are currently disturbed by unnecessary
> > jitter due to firing of such timers on them.
> > 
> > This patch will affine timers to online CPUs which are not full dynticks
> > in NOHZ_FULL configured systems. It should not introduce overhead in
> > nohz full off case due to static keys.
> > 
> > Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> > Signed-off by: Vatika Harlalka <vatikaharlalka@gmail.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Chris Metcalf <cmetcalf@ezchip.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > ---
> >  include/linux/tick.h | 9 ++++++++-
> >  kernel/sched/core.c  | 7 +++++--
> >  2 files changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index 3741ba1..51e6493 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -143,13 +143,20 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
> >  	if (tick_nohz_full_enabled())
> >  		cpumask_or(mask, mask, tick_nohz_full_mask);
> >  }
> > -
> > +static inline int housekeeping_any_cpu(void)
> > +{
> > +	return cpumask_any_and(housekeeping_mask, cpu_online_mask);
> > +}
> >  extern void __tick_nohz_full_check(void);
> >  extern void tick_nohz_full_kick(void);
> >  extern void tick_nohz_full_kick_cpu(int cpu);
> >  extern void tick_nohz_full_kick_all(void);
> >  extern void __tick_nohz_task_switch(struct task_struct *tsk);
> >  #else
> > +static inline int housekeeping_any_cpu(void)
> > +{
> > +	return smp_processor_id();
> > +}
> >  static inline bool tick_nohz_full_enabled(void) { return false; }
> >  static inline bool tick_nohz_full_cpu(int cpu) { return false; }
> >  static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9917c96..4fd42e4 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -623,18 +623,21 @@ int get_nohz_timer_target(void)
> >  	int i, cpu = smp_processor_id();
> >  	struct sched_domain *sd;
> >  
> > -	if (!idle_cpu(cpu))
> > +	if (!idle_cpu(cpu) && is_housekeeping_cpu(cpu))
> >  		return cpu;
> >  
> >  	rcu_read_lock();
> >  	for_each_domain(cpu, sd) {
> >  		for_each_cpu(i, sched_domain_span(sd)) {
> > -			if (!idle_cpu(i)) {
> > +			if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {
> >  				cpu = i;
> >  				goto unlock;
> >  			}
> >  		}
> >  	}
> > +
> > +	if (!is_housekeeping_cpu(cpu))
> > +		cpu = housekeeping_any_cpu();
> >  unlock:
> >  	rcu_read_unlock();
> >  	return cpu;
> 
> So I almost applied this yesterday, but had the following question: what ensures 
> that housekeeping_mask isn't empty? If it's empty then housekeeping_any_cpu() 
> returns cpumask_any_and() of an empty cpumask - which returns an out of range 
> index AFAICS - which will crash and burn in:
> 
> kernel/time/hrtimer.c:  return &per_cpu(hrtimer_bases, get_nohz_timer_target());
> kernel/time/timer.c:    return per_cpu_ptr(&tvec_bases, get_nohz_timer_target());
> 
> housekeeping_mask itself is derived from tick_nohz_full_mask (it's the inverse of 
> it in essence), and tick_nohz_full_mask is set via two methods, either via a boot 
> parameter:
> 
>         if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
> 
> in tick_nohz_full_setup(). What ensures here that tick_nohz_full_mask is not 
> completely full - making housekeeping_mask empty?
> 
> The other method is via CONFIG_NO_HZ_FULL_ALL:
> 
>         cpumask_setall(tick_nohz_full_mask);
> 
> here it's fully set - triggering the bug I'm worried about. So what am I missing, 
> what prevents CONFIG_NO_HZ_FULL_ALL from crashing?

The boot CPU is excluded from tick_nohz_full_mask in tick_nohz_init(),
which is called from tick_init() which is called from start_kernel()
shortly after rcu_init():

	cpu = smp_processor_id();

	if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) {
		pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
	}

This happens after the call to tick_nohz_init_all() that does the
cpumask_setall() that you called out above.

Or is a recent patch that I missed changing this?

							Thanx, Paul