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authorThomas Gleixner <tglx@linutronix.de>2015-05-26 22:50:33 +0000
committerThomas Gleixner <tglx@linutronix.de>2015-06-19 15:18:28 +0200
commitbc7a34b8b9ebfb0f4b8a35a72a0b134fd6c5ef50 (patch)
treef6324a2a7742e56740e9cc08d9636865ee72ec89 /kernel/time/timer.c
parentc74441a17eb975b604e339ca6c11b9ab9aaca11f (diff)
downloadlinux-sh-bc7a34b8b9ebfb0f4b8a35a72a0b134fd6c5ef50.tar.gz
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>
Diffstat (limited to 'kernel/time/timer.c')
-rw-r--r--kernel/time/timer.c59
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;
}
}