[PATCH] hrtimer: avoid retrigger_next_event IPI

[Marcelo Tosatti]

Setting the realtime clock triggers an IPI to all CPUs to reprogram
hrtimers.

However, only base, boottime and tai clocks have their offsets updated
(and therefore potentially require a reprogram).

If the CPU is a nohz_full one, check if it only has 
monotonic active timers, and in that case update the 
realtime base offsets, skipping the IPI.

This reduces interruptions to nohz_full CPUs.

Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com>

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 743c852e10f2..b42b1a434b22 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -853,6 +853,28 @@ static void hrtimer_reprogram(struct hrtimer *timer, bool 
reprogram)
        tick_program_event(expires, 1);
 }
 
+#define CLOCK_SET_BASES ((1U << HRTIMER_BASE_REALTIME)|                \
+                        (1U << HRTIMER_BASE_REALTIME_SOFT)|    \
+                        (1U << HRTIMER_BASE_BOOTTIME)|         \
+                        (1U << HRTIMER_BASE_BOOTTIME_SOFT)|    \
+                        (1U << HRTIMER_BASE_TAI)|              \
+                        (1U << HRTIMER_BASE_TAI_SOFT))
+
+static bool need_reprogram_timer(struct hrtimer_cpu_base *cpu_base)
+{
+       unsigned int active = 0;
+
+       if (!cpu_base->softirq_activated)
+               active = cpu_base->active_bases & HRTIMER_ACTIVE_SOFT;
+
+       active = active | (cpu_base->active_bases & HRTIMER_ACTIVE_HARD);
+
+       if ((active & CLOCK_SET_BASES) == 0)
+               return false;
+
+       return true;
+}
+
 /*
  * Clock realtime was set
  *
@@ -867,9 +889,41 @@ static void hrtimer_reprogram(struct hrtimer *timer, bool 
reprogram)
 void clock_was_set(void)
 {
 #ifdef CONFIG_HIGH_RES_TIMERS
-       /* Retrigger the CPU local events everywhere */
-       on_each_cpu(retrigger_next_event, NULL, 1);
+       cpumask_var_t mask;
+       int cpu;
+
+       if (!tick_nohz_full_enabled()) {
+               /* Retrigger the CPU local events everywhere */
+               on_each_cpu(retrigger_next_event, NULL, 1);
+               goto set_timerfd;
+       }
+
+       if (!zalloc_cpumask_var(&mask, GFP_KERNEL)) {
+               on_each_cpu(retrigger_next_event, NULL, 1);
+               goto set_timerfd;
+       }
+
+       /* Avoid interrupting nohz_full CPUs if possible */
+       preempt_disable();
+       for_each_online_cpu(cpu) {
+               if (tick_nohz_full_cpu(cpu)) {
+                       unsigned long flags;
+                       struct hrtimer_cpu_base *cpu_base = 
&per_cpu(hrtimer_bases, cpu);
+
+                       raw_spin_lock_irqsave(&cpu_base->lock, flags);
+                       if (need_reprogram_timer(cpu_base))
+                               cpumask_set_cpu(cpu, mask);
+                       else
+                               hrtimer_update_base(cpu_base);
+                       raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
+               }
+       }
+
+       smp_call_function_many(mask, retrigger_next_event, NULL, 1);
+       preempt_enable();
+       free_cpumask_var(mask);
 #endif
+set_timerfd:
        timerfd_clock_was_set();
 }