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

[Ingo Molnar <mingo@kernel.org>]

* 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?

Thanks,

	Ingo