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Subject[PATCH v6 01/36] timers: Use static keys for migrate_enable/nohz_active

From: Thomas Gleixner <tglx@linutronix.de>

The members migrate_enable and nohz_active in the timer/hrtimer per CPU
bases have been introduced to avoid accessing global variables for these
decisions.

Still that results in a (cache hot) load and conditional branch, which can
be avoided by using static keys.

Implement it with static keys and optimize for the most critical case of
high performance networking which tends to disable the timer migration
functionality.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: keescook@chromium.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20171221104205.7269-2-anna-maria@linutronix.de
---

V6: Updated to latest changes in mainline and timers/urgent. Addressed Frederics
review comments.

---
include/linux/hrtimer.h | 4 --
kernel/time/hrtimer.c | 17 ++-------
kernel/time/tick-internal.h | 19 ++++++----
kernel/time/tick-sched.c | 2 -
kernel/time/timer.c | 83 ++++++++++++++++++++++----------------------
5 files changed, 60 insertions(+), 65 deletions(-)

--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -153,8 +153,6 @@ enum hrtimer_base_type {
* @cpu: cpu number
* @active_bases: Bitfield to mark bases with active timers
* @clock_was_set_seq: Sequence counter of clock was set events
- * @migration_enabled: The migration of hrtimers to other cpus is enabled
- * @nohz_active: The nohz functionality is enabled
* @expires_next: absolute time of the next event which was scheduled
* via clock_set_next_event()
* @next_timer: Pointer to the first expiring timer
@@ -178,8 +176,6 @@ struct hrtimer_cpu_base {
unsigned int cpu;
unsigned int active_bases;
unsigned int clock_was_set_seq;
- bool migration_enabled;
- bool nohz_active;
#ifdef CONFIG_HIGH_RES_TIMERS
unsigned int in_hrtirq : 1,
hres_active : 1,
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -178,23 +178,16 @@ hrtimer_check_target(struct hrtimer *tim
#endif
}

-#ifdef CONFIG_NO_HZ_COMMON
-static inline
-struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base,
- int pinned)
-{
- if (pinned || !base->migration_enabled)
- return base;
- return &per_cpu(hrtimer_bases, get_nohz_timer_target());
-}
-#else
static inline
struct hrtimer_cpu_base *get_target_base(struct hrtimer_cpu_base *base,
int pinned)
{
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+ if (static_branch_likely(&timers_migration_enabled) && !pinned)
+ return &per_cpu(hrtimer_bases, get_nohz_timer_target());
+#endif
return base;
}
-#endif

/*
* We switch the timer base to a power-optimized selected CPU target,
@@ -969,7 +962,7 @@ void hrtimer_start_range_ns(struct hrtim
* Kick to reschedule the next tick to handle the new timer
* on dynticks target.
*/
- if (new_base->cpu_base->nohz_active)
+ if (is_timers_nohz_active())
wake_up_nohz_cpu(new_base->cpu_base->cpu);
} else {
hrtimer_reprogram(timer, new_base);
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -150,14 +150,19 @@ static inline void tick_nohz_init(void)

#ifdef CONFIG_NO_HZ_COMMON
extern unsigned long tick_nohz_active;
-#else
+extern void timers_update_nohz(void);
+extern struct static_key_false timers_nohz_active;
+static inline bool is_timers_nohz_active(void)
+{
+ return static_branch_likely(&timers_nohz_active);
+}
+# ifdef CONFIG_SMP
+extern struct static_key_false timers_migration_enabled;
+# endif
+#else /* CONFIG_NO_HZ_COMMON */
+static inline void timers_update_nohz(void) { }
#define tick_nohz_active (0)
-#endif
-
-#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
-extern void timers_update_migration(bool update_nohz);
-#else
-static inline void timers_update_migration(bool update_nohz) { }
+static inline bool is_timers_nohz_active(void) { return false; }
#endif

DECLARE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases);
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1107,7 +1107,7 @@ static inline void tick_nohz_activate(st
ts->nohz_mode = mode;
/* One update is enough */
if (!test_and_set_bit(0, &tick_nohz_active))
- timers_update_migration(true);
+ timers_update_nohz();
}

/**
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -200,8 +200,6 @@ struct timer_base {
unsigned long clk;
unsigned long next_expiry;
unsigned int cpu;
- bool migration_enabled;
- bool nohz_active;
bool is_idle;
bool must_forward_clk;
DECLARE_BITMAP(pending_map, WHEEL_SIZE);
@@ -210,45 +208,57 @@ struct timer_base {

static DEFINE_PER_CPU(struct timer_base, timer_bases[NR_BASES]);

-#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+#ifdef CONFIG_NO_HZ_COMMON
+
+DEFINE_STATIC_KEY_FALSE(timers_nohz_active);
+static DEFINE_MUTEX(timer_keys_mutex);
+
+static void timer_update_keys(struct work_struct *work);
+static DECLARE_WORK(timer_update_work, timer_update_keys);
+
+#ifdef CONFIG_SMP
unsigned int sysctl_timer_migration = 1;

-void timers_update_migration(bool update_nohz)
+DEFINE_STATIC_KEY_FALSE(timers_migration_enabled);
+
+static void timers_update_migration(void)
{
- bool on = sysctl_timer_migration && tick_nohz_active;
- unsigned int cpu;
+ if (sysctl_timer_migration && tick_nohz_active)
+ static_branch_enable(&timers_migration_enabled);
+ else
+ static_branch_disable(&timers_migration_enabled);
+}
+#else
+static inline void timers_update_migration(void) { }
+#endif /* !CONFIG_SMP */

- /* Avoid the loop, if nothing to update */
- if (this_cpu_read(timer_bases[BASE_STD].migration_enabled) == on)
- return;
+static void timer_update_keys(struct work_struct *work)
+{
+ mutex_lock(&timer_keys_mutex);
+ timers_update_migration();
+ static_branch_enable(&timers_nohz_active);
+ mutex_unlock(&timer_keys_mutex);
+}

- for_each_possible_cpu(cpu) {
- per_cpu(timer_bases[BASE_STD].migration_enabled, cpu) = on;
- per_cpu(timer_bases[BASE_DEF].migration_enabled, cpu) = on;
- per_cpu(hrtimer_bases.migration_enabled, cpu) = on;
- if (!update_nohz)
- continue;
- per_cpu(timer_bases[BASE_STD].nohz_active, cpu) = true;
- per_cpu(timer_bases[BASE_DEF].nohz_active, cpu) = true;
- per_cpu(hrtimer_bases.nohz_active, cpu) = true;
- }
+void timers_update_nohz(void)
+{
+ schedule_work(&timer_update_work);
}

int timer_migration_handler(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp,
loff_t *ppos)
{
- static DEFINE_MUTEX(mutex);
int ret;

- mutex_lock(&mutex);
+ mutex_lock(&timer_keys_mutex);
ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
if (!ret && write)
- timers_update_migration(false);
- mutex_unlock(&mutex);
+ timers_update_migration();
+ mutex_unlock(&timer_keys_mutex);
return ret;
}
-#endif
+#endif /* NO_HZ_COMMON */

static unsigned long round_jiffies_common(unsigned long j, int cpu,
bool force_up)
@@ -534,7 +544,7 @@ static void
static void
trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
{
- if (!IS_ENABLED(CONFIG_NO_HZ_COMMON) || !base->nohz_active)
+ if (!is_timers_nohz_active())
return;

/*
@@ -849,21 +859,20 @@ static inline struct timer_base *get_tim
return get_timer_cpu_base(tflags, tflags & TIMER_CPUMASK);
}

-#ifdef CONFIG_NO_HZ_COMMON
static inline struct timer_base *
get_target_base(struct timer_base *base, unsigned tflags)
{
-#ifdef CONFIG_SMP
- if ((tflags & TIMER_PINNED) || !base->migration_enabled)
- return get_timer_this_cpu_base(tflags);
- return get_timer_cpu_base(tflags, get_nohz_timer_target());
-#else
- return get_timer_this_cpu_base(tflags);
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+ if (static_branch_likely(&timers_migration_enabled) &&
+ !(tflags & TIMER_PINNED))
+ return get_timer_cpu_base(tflags, get_nohz_timer_target());
#endif
+ return get_timer_this_cpu_base(tflags);
}

static inline void forward_timer_base(struct timer_base *base)
{
+#ifdef CONFIG_NO_HZ_COMMON
unsigned long jnow;

/*
@@ -887,16 +896,8 @@ static inline void forward_timer_base(st
base->clk = jnow;
else
base->clk = base->next_expiry;
-}
-#else
-static inline struct timer_base *
-get_target_base(struct timer_base *base, unsigned tflags)
-{
- return get_timer_this_cpu_base(tflags);
-}
-
-static inline void forward_timer_base(struct timer_base *base) { }
#endif
+}


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