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Date: Mon, 27 Apr 2015 19:41:36 -0700
From: Joonwoo Park <joonwoop@...eaurora.org>
To: tglx@...utronix.de
CC: skannan@...eaurora.org, linux-kernel@...r.kernel.org,
John Stultz <john.stultz@...aro.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] timer: make deferrable cpu unbound timers really
not bound to a cpu
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@...utronix.de>
> CC: John Stultz <john.stultz@...aro.org>
> CC: Tejun Heo <tj@...nel.org>
> Signed-off-by: Joonwoo Park <joonwoop@...eaurora.org>
> ---
> 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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