timer: Reduce timer migration overhead if disabled

Eric reported that the timer_migration sysctl is not really nice
performance wise as it needs to check at every timer insertion whether
the feature is enabled or not. Further the check does not live in the
timer code, so we have an extra function call which checks an extra
cache line to figure out that it is disabled.

We can do better and store that information in the per cpu (hr)timer
bases. I pondered to use a static key, but that's a nightmare to
update from the nohz code and the timer base cache line is hot anyway
when we select a timer base.

The old logic enabled the timer migration unconditionally if
CONFIG_NO_HZ was set even if nohz was disabled on the kernel command
line.

With this modification, we start off with migration disabled. The user
visible sysctl is still set to enabled. If the kernel switches to NOHZ
migration is enabled, if the user did not disable it via the sysctl
prior to the switch. If nohz=off is on the kernel command line,
migration stays disabled no matter what.

Before:
  47.76%  hog       [.] main
  14.84%  [kernel]  [k] _raw_spin_lock_irqsave
   9.55%  [kernel]  [k] _raw_spin_unlock_irqrestore
   6.71%  [kernel]  [k] mod_timer
   6.24%  [kernel]  [k] lock_timer_base.isra.38
   3.76%  [kernel]  [k] detach_if_pending
   3.71%  [kernel]  [k] del_timer
   2.50%  [kernel]  [k] internal_add_timer
   1.51%  [kernel]  [k] get_nohz_timer_target
   1.28%  [kernel]  [k] __internal_add_timer
   0.78%  [kernel]  [k] timerfn
   0.48%  [kernel]  [k] wake_up_nohz_cpu

After:
  48.10%  hog       [.] main
  15.25%  [kernel]  [k] _raw_spin_lock_irqsave
   9.76%  [kernel]  [k] _raw_spin_unlock_irqrestore
   6.50%  [kernel]  [k] mod_timer
   6.44%  [kernel]  [k] lock_timer_base.isra.38
   3.87%  [kernel]  [k] detach_if_pending
   3.80%  [kernel]  [k] del_timer
   2.67%  [kernel]  [k] internal_add_timer
   1.33%  [kernel]  [k] __internal_add_timer
   0.73%  [kernel]  [k] timerfn
   0.54%  [kernel]  [k] wake_up_nohz_cpu


Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Joonwoo Park <joonwoop@codeaurora.org>
Cc: Wenbo Wang <wenbo.wang@memblaze.com>
Link: http://lkml.kernel.org/r/20150526224512.127050787@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 54 insertions, 5 deletions

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 3398d93c74a7..343142ed996a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -85,6 +85,7 @@ struct tvec_base {
         unsigned long active_timers;
         unsigned long all_timers;
         int cpu;
+        bool migration_enabled;
         struct tvec_root tv1;
         struct tvec tv2;
         struct tvec tv3;
@@ -95,6 +96,54 @@ struct tvec_base {
 
 static DEFINE_PER_CPU(struct tvec_base, tvec_bases);
 
+#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
+unsigned int sysctl_timer_migration = 1;
+
+void timers_update_migration(void)
+{
+        bool on = sysctl_timer_migration && tick_nohz_active;
+        unsigned int cpu;
+
+        /* Avoid the loop, if nothing to update */
+        if (this_cpu_read(tvec_bases.migration_enabled) == on)
+                return;
+
+        for_each_possible_cpu(cpu) {
+                per_cpu(tvec_bases.migration_enabled, cpu) = on;
+                per_cpu(hrtimer_bases.migration_enabled, cpu) = on;
+        }
+}
+
+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);
+        ret = proc_dointvec(table, write, buffer, lenp, ppos);
+        if (!ret && write)
+                timers_update_migration();
+        mutex_unlock(&mutex);
+        return ret;
+}
+
+static inline struct tvec_base *get_target_base(struct tvec_base *base,
+                                                int pinned)
+{
+        if (pinned || !base->migration_enabled)
+                return this_cpu_ptr(&tvec_bases);
+        return per_cpu_ptr(&tvec_bases, get_nohz_timer_target());
+}
+#else
+static inline struct tvec_base *get_target_base(struct tvec_base *base,
+                                                int pinned)
+{
+        return this_cpu_ptr(&tvec_bases);
+}
+#endif
+
 static unsigned long round_jiffies_common(unsigned long j, int cpu,
                 bool force_up)
 {
@@ -716,11 +765,11 @@ static struct tvec_base *lock_timer_base(struct timer_list *timer,
 
 static inline int
 __mod_timer(struct timer_list *timer, unsigned long expires,
-                                                bool pending_only, int pinned)
+            bool pending_only, int pinned)
 {
         struct tvec_base *base, *new_base;
         unsigned long flags;
-        int ret = 0 , cpu;
+        int ret = 0;
 
         timer_stats_timer_set_start_info(timer);
         BUG_ON(!timer->function);
@@ -733,8 +782,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
 
         debug_activate(timer, expires);
 
-        cpu = get_nohz_timer_target(pinned);
-        new_base = per_cpu_ptr(&tvec_bases, cpu);
+        new_base = get_target_base(base, pinned);
 
         if (base != new_base) {
                 /*
@@ -751,7 +799,8 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
                         spin_unlock(&base->lock);
                         base = new_base;
                         spin_lock(&base->lock);
-                        timer->flags = (timer->flags & ~TIMER_BASEMASK) | cpu;
+                        timer->flags &= ~TIMER_BASEMASK;
+                        timer->flags |= base->cpu;
                 }
         }