Re: [PATCH 2/2] timer: make deferrable cpu unbound timers really not bound to a cpu

[Joonwoo Park]

Thomas, I made some clean up.  Will much appreciate if you can give me some feedback on this.

Thanks,
Joonwoo

On 04/27/2015 07:39 PM, Joonwoo Park wrote:
> When a deferrable work (INIT_DEFERRABLE_WORK, etc.) is queued via
> queue_delayed_work() it's probably intended to run the work item on any
> CPU that isn't idle. However, we queue the work to run at a later time
> by starting a deferrable timer that binds to whatever CPU the work is
> queued on which is same with queue_delayed_work_on(smp_processor_id())
> effectively.
> 
> As a result WORK_CPU_UNBOUND work items aren't really cpu unbound now.
> In fact this is perfectly fine with UP kernel and also won't affect much a
> system without dyntick with SMP kernel too as every cpus run timers
> periodically.  But on SMP systems with dyntick current implementation leads
> deferrable timers not very scalable because the timer's base which has
> queued the deferrable timer won't wake up till next non-deferrable timer
> expires even though there are possible other non idle cpus are running
> which are able to run expired deferrable timers.
> 
> The deferrable work is a good example of the current implementation's
> victim like below.
> 
> INIT_DEFERRABLE_WORK(&dwork, fn);
> CPU 0                                 CPU 1
> queue_delayed_work(wq, &dwork, HZ);
>     queue_delayed_work_on(WORK_CPU_UNBOUND);
>         ...
> 	__mod_timer() -> queues timer to the
> 			 current cpu's timer
> 			 base.
> 	...
> tick_nohz_idle_enter() -> cpu enters idle.
> A second later
> cpu 0 is now in idle.                 cpu 1 exits idle or wasn't in idle so
>                                       now it's in active but won't
> cpu 0 won't wake up till next         handle cpu unbound deferrable timer
> non-deferrable timer expires.         as it's in cpu 0's timer base.
> 
> To make all cpu unbound deferrable timers are scalable, introduce a common
> timer base which is only for cpu unbound deferrable timers to make those
> are indeed cpu unbound so that can be scheduled by tick_do_timer_cpu.
> This common timer fixes scalability issue of delayed work and all other cpu
> unbound deferrable timer using implementations.
> 
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: John Stultz <john.stultz@xxxxxxxxxx>
> CC: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>
> ---
> Changes in v3:
>  * Make only tick_do_timer_cpu to run deferral timer wheel to reduce cache bouncing.
> 
> Changes in v4:
>  * Kill CONFIG_SMP ifdefry.
>  * Allocate and initialize tvec_base_deferrable at compile time.
>  * Pin pinned deferrable timer. 
>  * s/deferral/deferrable/
> 
>  include/linux/timer.h |  14 ++++++-
>  kernel/time/timer.c   | 103 ++++++++++++++++++++++++++++++++++++++++----------
>  2 files changed, 97 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/timer.h b/include/linux/timer.h
> index 8c5a197..45847ca 100644
> --- a/include/linux/timer.h
> +++ b/include/linux/timer.h
> @@ -34,6 +34,9 @@ struct timer_list {
>  };
>  
>  extern struct tvec_base boot_tvec_bases;
> +#ifdef CONFIG_SMP
> +extern struct tvec_base tvec_base_deferrable;
> +#endif
>  
>  #ifdef CONFIG_LOCKDEP
>  /*
> @@ -70,12 +73,21 @@ extern struct tvec_base boot_tvec_bases;
>  
>  #define TIMER_FLAG_MASK			0x3LU
>  
> +#ifdef CONFIG_SMP
> +#define __TIMER_BASE(_flags) \
> +	((_flags) & TIMER_DEFERRABLE ? \
> +	 (unsigned long)&tvec_base_deferrable + (_flags) : \
> +	 (unsigned long)&boot_tvec_bases + (_flags))
> +#else
> +#define __TIMER_BASE(_flags) ((unsigned long)&boot_tvec_bases + (_flags))
> +#endif
> +
>  #define __TIMER_INITIALIZER(_function, _expires, _data, _flags) { \
>  		.entry = { .prev = TIMER_ENTRY_STATIC },	\
>  		.function = (_function),			\
>  		.expires = (_expires),				\
>  		.data = (_data),				\
> -		.base = (void *)((unsigned long)&boot_tvec_bases + (_flags)), \
> +		.base = (void *)(__TIMER_BASE(_flags)),		\
>  		.slack = -1,					\
>  		__TIMER_LOCKDEP_MAP_INITIALIZER(		\
>  			__FILE__ ":" __stringify(__LINE__))	\
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index e5d5733c..133e94a 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -49,6 +49,8 @@
>  #include <asm/timex.h>
>  #include <asm/io.h>
>  
> +#include "tick-internal.h"
> +
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/timer.h>
>  
> @@ -103,6 +105,9 @@ struct tvec_base boot_tvec_bases;
>  EXPORT_SYMBOL(boot_tvec_bases);
>  
>  static DEFINE_PER_CPU(struct tvec_base *, tvec_bases) = &boot_tvec_bases;
> +#ifdef CONFIG_SMP
> +struct tvec_base tvec_base_deferrable;
> +#endif
>  
>  /* Functions below help us manage 'deferrable' flag */
>  static inline unsigned int tbase_get_deferrable(struct tvec_base *base)
> @@ -662,10 +667,63 @@ static inline void debug_assert_init(struct timer_list *timer)
>  	debug_timer_assert_init(timer);
>  }
>  
> +#ifdef CONFIG_SMP
> +static inline struct tvec_base *__get_timer_base(unsigned int flags)
> +{
> +	if (flags & TIMER_DEFERRABLE)
> +		return &tvec_base_deferrable;
> +	else
> +		return raw_cpu_read(tvec_bases);
> +}
> +
> +static inline bool is_deferrable_timer_base(struct tvec_base *base)
> +{
> +	return base == &tvec_base_deferrable;
> +}
> +
> +static inline void __run_timers(struct tvec_base *base);
> +static inline void __run_deferrable_timers(void)
> +{
> +	if (smp_processor_id() == tick_do_timer_cpu &&
> +	    time_after_eq(jiffies, tvec_base_deferrable.timer_jiffies))
> +		__run_timers(&tvec_base_deferrable);
> +}
> +
> +static void __init init_timer_cpu(struct tvec_base *base, int cpu);
> +static inline void init_deferrable_timer(void)
> +{
> +	init_timer_cpu(&tvec_base_deferrable, NR_CPUS);
> +}
> +#else
> +static inline struct tvec_base *__get_timer_base(unsigned int flags)
> +{
> +	return raw_cpu_read(tvec_bases);
> +}
> +
> +static inline bool is_deferrable_timer_base(struct tvec_base *base)
> +{
> +	return false;
> +}
> +
> +static inline void __run_deferrable_timers(void)
> +{
> +}
> +
> +static inline void init_deferrable_timer(void)
> +{
> +	/*
> +	 * initialize cpu unbound deferrable timer base only when CONFIG_SMP.
> +	 * UP kernel handles the timers with cpu 0 timer base.
> +	 */
> +}
> +#endif
> +
>  static void do_init_timer(struct timer_list *timer, unsigned int flags,
>  			  const char *name, struct lock_class_key *key)
>  {
> -	struct tvec_base *base = raw_cpu_read(tvec_bases);
> +	struct tvec_base *base;
> +
> +	base = __get_timer_base(flags);
>  
>  	timer->entry.next = NULL;
>  	timer->base = (void *)((unsigned long)base | flags);
> @@ -787,24 +845,26 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
>  
>  	debug_activate(timer, expires);
>  
> -	cpu = get_nohz_timer_target(pinned);
> -	new_base = per_cpu(tvec_bases, cpu);
> +	if (!is_deferrable_timer_base(base) || pinned == TIMER_PINNED) {
> +		cpu = get_nohz_timer_target(pinned);
> +		new_base = per_cpu(tvec_bases, cpu);
>  
> -	if (base != new_base) {
> -		/*
> -		 * We are trying to schedule the timer on the local CPU.
> -		 * However we can't change timer's base while it is running,
> -		 * otherwise del_timer_sync() can't detect that the timer's
> -		 * handler yet has not finished. This also guarantees that
> -		 * the timer is serialized wrt itself.
> -		 */
> -		if (likely(base->running_timer != timer)) {
> -			/* See the comment in lock_timer_base() */
> -			timer_set_base(timer, NULL);
> -			spin_unlock(&base->lock);
> -			base = new_base;
> -			spin_lock(&base->lock);
> -			timer_set_base(timer, base);
> +		if (base != new_base) {
> +			/*
> +			 * We are trying to schedule the timer on the local CPU.
> +			 * However we can't change timer's base while it is
> +			 * running, otherwise del_timer_sync() can't detect that
> +			 * the timer's handler yet has not finished. This also
> +			 * guarantees that the timer is serialized wrt itself.
> +			 */
> +			if (likely(base->running_timer != timer)) {
> +				/* See the comment in lock_timer_base() */
> +				timer_set_base(timer, NULL);
> +				spin_unlock(&base->lock);
> +				base = new_base;
> +				spin_lock(&base->lock);
> +				timer_set_base(timer, base);
> +			}
>  		}
>  	}
>  
> @@ -1411,6 +1471,8 @@ static void run_timer_softirq(struct softirq_action *h)
>  
>  	hrtimer_run_pending();
>  
> +	__run_deferrable_timers();
> +
>  	if (time_after_eq(jiffies, base->timer_jiffies))
>  		__run_timers(base);
>  }
> @@ -1623,7 +1685,8 @@ static void __init init_timer_cpu(struct tvec_base *base, int cpu)
>  	BUG_ON(base != tbase_get_base(base));
>  
>  	base->cpu = cpu;
> -	per_cpu(tvec_bases, cpu) = base;
> +	if (cpu != NR_CPUS)
> +		per_cpu(tvec_bases, cpu) = base;
>  	spin_lock_init(&base->lock);
>  
>  	for (j = 0; j < TVN_SIZE; j++) {
> @@ -1655,6 +1718,8 @@ static void __init init_timer_cpus(void)
>  
>  		init_timer_cpu(base, cpu);
>  	}
> +
> +	init_deferrable_timer();
>  }
>  
>  void __init init_timers(void)
> 
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