[PATCH v4 16/16] timer: Always queue timers on the local CPU

From: Anna-Maria Behnsen
Date: Fri Nov 04 2022 - 10:59:24 EST


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 seperate storage
and eventually pulled at expiry time to a remote CPU.

When a timer is added via add_timer_on(), TIMER_PINNED flag is required to
ensure it expires on the specified CPU. Otherwise it will be enqueued in
the global timer base which could be expired by a remote CPU. WARN_ONCE()
is added to prevent misuse.

Originally-by: Richard Cochran (linutronix GmbH) <richardcochran@xxxxxxxxx>
Signed-off-by: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
---
v4: Update comment about TIMER_PINNED flag (heristic is removed)
---
include/linux/timer.h | 5 ++---
kernel/time/timer.c | 17 +++++------------
2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 648f00105f58..181a3d3fa45a 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -50,9 +50,8 @@ 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.
+ * @TIMER_PINNED: A pinned timer 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
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 214397d84747..bdff4be066e4 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -947,17 +947,6 @@ 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);
@@ -1093,7 +1082,7 @@ __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) {
/*
@@ -1228,6 +1217,10 @@ void add_timer_on(struct timer_list *timer, int cpu)

BUG_ON(timer_pending(timer) || !timer->function);

+ WARN_ONCE(!(timer->flags & TIMER_PINNED), "TIMER_PINNED flag for "
+ "add_timer_on() is missing: timer=%p function=%ps",
+ timer, timer->function);
+
new_base = get_timer_cpu_base(timer->flags, cpu);

/*
--
2.30.2