[PATCH v7 21/21] timer: Always queue timers on the local CPU

Anna-Maria Behnsen
2023-05-24T07:08:40Z

The timer pull model is in place so we can remove the heuristics which try
to guess the best target CPU at enqueue/modification time.

All non pinned timers are queued on the local CPU in the separate storage
and eventually pulled at expiry time to a remote CPU.

Originally-by: Richard Cochran (linutronix GmbH) <richardcochran@gmail.com>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
---
v6:
- Update TIMER_PINNED flag description.

v5:
- Move WARN_ONCE() in add_timer_on() into a previous patch
- Fold crystallball magic related hunks into this patch

v4: Update comment about TIMER_PINNED flag (heristic is removed)
---
include/linux/timer.h | 14 ++++----------
kernel/time/timer.c | 42 ++++++++++++++++++++----------------------
2 files changed, 24 insertions(+), 32 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 6f96661480dd..c39ae2ad0035 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -50,16 +50,10 @@ struct timer_list {
* workqueue locking issues. It's not meant for executing random crap
* with interrupts disabled. Abuse is monitored!
*
- * @TIMER_PINNED: A pinned timer will not be affected by any timer
- * placement heuristics (like, NOHZ) and will always expire on the CPU
- * on which the timer was enqueued.
- *
- * Note: Because enqueuing of timers can migrate the timer from one
- * CPU to another, pinned timers are not guaranteed to stay on the
- * initialy selected CPU. They move to the CPU on which the enqueue
- * function is invoked via mod_timer() or add_timer(). If the timer
- * should be placed on a particular CPU, then add_timer_on() has to be
- * used.
+ * @TIMER_PINNED: A pinned timer will always expire on the CPU on which the
+ * timer was enqueued. When a particular CPU is required, add_timer_on()
+ * has to be used. Enqueue via mod_timer() and add_timer() is always done
+ * on the local CPU.
*/
#define TIMER_CPUMASK 0x0003FFFF
#define TIMER_MIGRATING 0x00040000
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 3a8c2c88f845..5602d411a70e 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -590,10 +590,13 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)

/*
* We might have to IPI the remote CPU if the base is idle and the
- * timer is not deferrable. If the other CPU is on the way to idle
- * then it can't set base->is_idle as we hold the base lock:
+ * timer is pinned. If it is a non pinned timer, it is only queued
+ * on the remote CPU, when timer was running during queueing. Then
+ * everything is handled by remote CPU anyway. If the other CPU is
+ * on the way to idle then it can't set base->is_idle as we hold
+ * the base lock:
*/
- if (base->is_idle)
+ if (base->is_idle && timer->flags & TIMER_PINNED)
wake_up_nohz_cpu(base->cpu);
}

@@ -941,17 +944,6 @@ static inline struct timer_base *get_timer_base(u32 tflags)
return get_timer_cpu_base(tflags, tflags & TIMER_CPUMASK);
}

-static inline struct timer_base *
-get_target_base(struct timer_base *base, unsigned 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)
{
unsigned long jnow = READ_ONCE(jiffies);
@@ -1103,7 +1095,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
if (!ret && (options & MOD_TIMER_PENDING_ONLY))
goto out_unlock;

- new_base = get_target_base(base, timer->flags);
+ new_base = get_timer_this_cpu_base(timer->flags);

if (base != new_base) {
/*
@@ -2202,8 +2194,14 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
*/
}

- /* We need to mark both bases in sync */
- base_local->is_idle = base_global->is_idle = is_idle;
+ /*
+ * base->is_idle information is required to wakeup a idle CPU when
+ * a new timer was enqueued. Only pinned timers could be enqueued
+ * remotely into a idle base. Therefore do maintain only
+ * base_local->is_idle information and ignore base_global->is_idle
+ * information.
+ */
+ base_local->is_idle = is_idle;

raw_spin_unlock(&base_global->lock);
raw_spin_unlock(&base_local->lock);
@@ -2233,13 +2231,13 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
void timer_clear_idle(void)
{
/*
- * We do this unlocked. The worst outcome is a remote enqueue sending
- * a pointless IPI, but taking the lock would just make the window for
- * sending the IPI a few instructions smaller for the cost of taking
- * the lock in the exit from idle path.
+ * We do this unlocked. The worst outcome is a remote pinned timer
+ * enqueue sending a pointless IPI, but taking the lock would just
+ * make the window for sending the IPI a few instructions smaller
+ * for the cost of taking the lock in the exit from idle
+ * path. Required for BASE_LOCAL only.
*/
__this_cpu_write(timer_bases[BASE_LOCAL].is_idle, false);
- __this_cpu_write(timer_bases[BASE_GLOBAL].is_idle, false);

/* Activate without holding the timer_base->lock */
tmigr_cpu_activate();
--
2.30.2