timers: Always queue timers on the local CPU

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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240221090548.36600-21-anna-maria@linutronix.de

diff --git a/include/linux/timer.h b/include/linux/timer.h
index 2be8be6dd317c0a06500c933a03b19ed9583ccb4..14a633ba61d6433400a488ff21d73172f3616281 100644 (file)
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -36,16 +36,10 @@
  * 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 3ed135c8de430725da71e8e00c89f4c8bd0b689a..4f4930da6448ba0e446ba72f03a07c67593923c9 100644 (file)
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -635,11 +635,16 @@ 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) {
+               WARN_ON_ONCE(!(timer->flags & TIMER_PINNED));
                wake_up_nohz_cpu(base->cpu);
+       }
 }
 
 /*
@@ -986,17 +991,6 @@ 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 basej)
 {
@@ -1151,7 +1145,7 @@ __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) {
                /*
@@ -2297,7 +2291,7 @@ static inline u64 __get_next_timer_interrupt(unsigned long basej, u64 basem,
                 * granularity skew (by design).
                 */
                if (!base_local->is_idle && time_after(nextevt, basej + 1)) {
-                       base_local->is_idle = base_global->is_idle = true;
+                       base_local->is_idle = true;
                        trace_timer_base_idle(true, base_local->cpu);
                }
                *idle = base_local->is_idle;
@@ -2363,13 +2357,13 @@ u64 timer_base_try_to_set_idle(unsigned long basej, u64 basem, bool *idle)
 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);
        trace_timer_base_idle(false, smp_processor_id());
 
        /* Activate without holding the timer_base->lock */