Merge branch 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull 'full dynticks' support from Ingo Molnar:
 "This tree from Frederic Weisbecker adds a new, (exciting! :-) core
  kernel feature to the timer and scheduler subsystems: 'full dynticks',
  or CONFIG_NO_HZ_FULL=y.

  This feature extends the nohz variable-size timer tick feature from
  idle to busy CPUs (running at most one task) as well, potentially
  reducing the number of timer interrupts significantly.

  This feature got motivated by real-time folks and the -rt tree, but
  the general utility and motivation of full-dynticks runs wider than
  that:

   - HPC workloads get faster: CPUs running a single task should be able
     to utilize a maximum amount of CPU power.  A periodic timer tick at
     HZ=1000 can cause a constant overhead of up to 1.0%.  This feature
     removes that overhead - and speeds up the system by 0.5%-1.0% on
     typical distro configs even on modern systems.

   - Real-time workload latency reduction: CPUs running critical tasks
     should experience as little jitter as possible.  The last remaining
     source of kernel-related jitter was the periodic timer tick.

   - A single task executing on a CPU is a pretty common situation,
     especially with an increasing number of cores/CPUs, so this feature
     helps desktop and mobile workloads as well.

  The cost of the feature is mainly related to increased timer
  reprogramming overhead when a CPU switches its tick period, and thus
  slightly longer to-idle and from-idle latency.

  Configuration-wise a third mode of operation is added to the existing
  two NOHZ kconfig modes:

   - CONFIG_HZ_PERIODIC: [formerly !CONFIG_NO_HZ], now explicitly named
     as a config option.  This is the traditional Linux periodic tick
     design: there's a HZ tick going on all the time, regardless of
     whether a CPU is idle or not.

   - CONFIG_NO_HZ_IDLE: [formerly CONFIG_NO_HZ=y], this turns off the
     periodic tick when a CPU enters idle mode.

   - CONFIG_NO_HZ_FULL: this new mode, in addition to turning off the
     tick when a CPU is idle, also slows the tick down to 1 Hz (one
     timer interrupt per second) when only a single task is running on a
     CPU.

  The .config behavior is compatible: existing !CONFIG_NO_HZ and
  CONFIG_NO_HZ=y settings get translated to the new values, without the
  user having to configure anything.  CONFIG_NO_HZ_FULL is turned off by
  default.

  This feature is based on a lot of infrastructure work that has been
  steadily going upstream in the last 2-3 cycles: related RCU support
  and non-periodic cputime support in particular is upstream already.

  This tree adds the final pieces and activates the feature.  The pull
  request is marked RFC because:

   - it's marked 64-bit only at the moment - the 32-bit support patch is
     small but did not get ready in time.

   - it has a number of fresh commits that came in after the merge
     window.  The overwhelming majority of commits are from before the
     merge window, but still some aspects of the tree are fresh and so I
     marked it RFC.

   - it's a pretty wide-reaching feature with lots of effects - and
     while the components have been in testing for some time, the full
     combination is still not very widely used.  That it's default-off
     should reduce its regression abilities and obviously there are no
     known regressions with CONFIG_NO_HZ_FULL=y enabled either.

   - the feature is not completely idempotent: there is no 100%
     equivalent replacement for a periodic scheduler/timer tick.  In
     particular there's ongoing work to map out and reduce its effects
     on scheduler load-balancing and statistics.  This should not impact
     correctness though, there are no known regressions related to this
     feature at this point.

   - it's a pretty ambitious feature that with time will likely be
     enabled by most Linux distros, and we'd like you to make input on
     its design/implementation, if you dislike some aspect we missed.
     Without flaming us to crisp! :-)

  Future plans:

   - there's ongoing work to reduce 1Hz to 0Hz, to essentially shut off
     the periodic tick altogether when there's a single busy task on a
     CPU.  We'd first like 1 Hz to be exposed more widely before we go
     for the 0 Hz target though.

   - once we reach 0 Hz we can remove the periodic tick assumption from
     nr_running>=2 as well, by essentially interrupting busy tasks only
     as frequently as the sched_latency constraints require us to do -
     once every 4-40 msecs, depending on nr_running.

  I am personally leaning towards biting the bullet and doing this in
  v3.10, like the -rt tree this effort has been going on for too long -
  but the final word is up to you as usual.

  More technical details can be found in Documentation/timers/NO_HZ.txt"

* 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (39 commits)
  sched: Keep at least 1 tick per second for active dynticks tasks
  rcu: Fix full dynticks' dependency on wide RCU nocb mode
  nohz: Protect smp_processor_id() in tick_nohz_task_switch()
  nohz_full: Add documentation.
  cputime_nsecs: use math64.h for nsec resolution conversion helpers
  nohz: Select VIRT_CPU_ACCOUNTING_GEN from full dynticks config
  nohz: Reduce overhead under high-freq idling patterns
  nohz: Remove full dynticks' superfluous dependency on RCU tree
  nohz: Fix unavailable tick_stop tracepoint in dynticks idle
  nohz: Add basic tracing
  nohz: Select wide RCU nocb for full dynticks
  nohz: Disable the tick when irq resume in full dynticks CPU
  nohz: Re-evaluate the tick for the new task after a context switch
  nohz: Prepare to stop the tick on irq exit
  nohz: Implement full dynticks kick
  nohz: Re-evaluate the tick from the scheduler IPI
  sched: New helper to prevent from stopping the tick in full dynticks
  sched: Kick full dynticks CPU that have more than one task enqueued.
  perf: New helper to prevent full dynticks CPUs from stopping tick
  perf: Kick full dynticks CPU if events rotation is needed
  ...

4 files changed, 360 insertions, 24 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 24510d84efd7..e4c07b0692bb 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -64,20 +64,88 @@ config GENERIC_CMOS_UPDATE
 if GENERIC_CLOCKEVENTS
 menu "Timers subsystem"
 
-# Core internal switch. Selected by NO_HZ / HIGH_RES_TIMERS. This is
+# Core internal switch. Selected by NO_HZ_COMMON / HIGH_RES_TIMERS. This is
 # only related to the tick functionality. Oneshot clockevent devices
 # are supported independ of this.
 config TICK_ONESHOT
         bool
 
-config NO_HZ
-        bool "Tickless System (Dynamic Ticks)"
+config NO_HZ_COMMON
+        bool
         depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
         select TICK_ONESHOT
+
+choice
+        prompt "Timer tick handling"
+        default NO_HZ_IDLE if NO_HZ
+
+config HZ_PERIODIC
+        bool "Periodic timer ticks (constant rate, no dynticks)"
+        help
+          This option keeps the tick running periodically at a constant
+          rate, even when the CPU doesn't need it.
+
+config NO_HZ_IDLE
+        bool "Idle dynticks system (tickless idle)"
+        depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
+        select NO_HZ_COMMON
+        help
+          This option enables a tickless idle system: timer interrupts
+          will only trigger on an as-needed basis when the system is idle.
+          This is usually interesting for energy saving.
+
+          Most of the time you want to say Y here.
+
+config NO_HZ_FULL
+        bool "Full dynticks system (tickless)"
+        # NO_HZ_COMMON dependency
+        depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
+        # We need at least one periodic CPU for timekeeping
+        depends on SMP
+        # RCU_USER_QS dependency
+        depends on HAVE_CONTEXT_TRACKING
+        # VIRT_CPU_ACCOUNTING_GEN dependency
+        depends on 64BIT
+        select NO_HZ_COMMON
+        select RCU_USER_QS
+        select RCU_NOCB_CPU
+        select VIRT_CPU_ACCOUNTING_GEN
+        select CONTEXT_TRACKING_FORCE
+        select IRQ_WORK
+        help
+         Adaptively try to shutdown the tick whenever possible, even when
+         the CPU is running tasks. Typically this requires running a single
+         task on the CPU. Chances for running tickless are maximized when
+         the task mostly runs in userspace and has few kernel activity.
+
+         You need to fill up the nohz_full boot parameter with the
+         desired range of dynticks CPUs.
+
+         This is implemented at the expense of some overhead in user <-> kernel
+         transitions: syscalls, exceptions and interrupts. Even when it's
+         dynamically off.
+
+         Say N.
+
+endchoice
+
+config NO_HZ_FULL_ALL
+       bool "Full dynticks system on all CPUs by default"
+       depends on NO_HZ_FULL
+       help
+         If the user doesn't pass the nohz_full boot option to
+         define the range of full dynticks CPUs, consider that all
+         CPUs in the system are full dynticks by default.
+         Note the boot CPU will still be kept outside the range to
+         handle the timekeeping duty.
+
+config NO_HZ
+        bool "Old Idle dynticks config"
+        depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
         help
-          This option enables a tickless system: timer interrupts will
-          only trigger on an as-needed basis both when the system is
-          busy and when the system is idle.
+          This is the old config entry that enables dynticks idle.
+          We keep it around for a little while to enforce backward
+          compatibility with older config files.
 
 config HIGH_RES_TIMERS
         bool "High Resolution Timer Support"
diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index 61d00a8cdf2f..206bbfb34e09 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -693,7 +693,8 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
                 bc->event_handler = tick_handle_oneshot_broadcast;
 
                 /* Take the do_timer update */
-                tick_do_timer_cpu = cpu;
+                if (!tick_nohz_full_cpu(cpu))
+                        tick_do_timer_cpu = cpu;
 
                 /*
                  * We must be careful here. There might be other CPUs
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 6176a3e45709..5d3fb100bc06 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -163,7 +163,10 @@ static void tick_setup_device(struct tick_device *td,
                  * this cpu:
                  */
                 if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
-                        tick_do_timer_cpu = cpu;
+                        if (!tick_nohz_full_cpu(cpu))
+                                tick_do_timer_cpu = cpu;
+                        else
+                                tick_do_timer_cpu = TICK_DO_TIMER_NONE;
                         tick_next_period = ktime_get();
                         tick_period = ktime_set(0, NSEC_PER_SEC / HZ);
                 }
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 225f8bf19095..bc67d4245e1d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -21,11 +21,15 @@
 #include <linux/sched.h>
 #include <linux/module.h>
 #include <linux/irq_work.h>
+#include <linux/posix-timers.h>
+#include <linux/perf_event.h>
 
 #include <asm/irq_regs.h>
 
 #include "tick-internal.h"
 
+#include <trace/events/timer.h>
+
 /*
  * Per cpu nohz control structure
  */
@@ -104,7 +108,7 @@ static void tick_sched_do_timer(ktime_t now)
 {
         int cpu = smp_processor_id();
 
-#ifdef CONFIG_NO_HZ
+#ifdef CONFIG_NO_HZ_COMMON
         /*
          * Check if the do_timer duty was dropped. We don't care about
          * concurrency: This happens only when the cpu in charge went
@@ -112,7 +116,8 @@ static void tick_sched_do_timer(ktime_t now)
          * this duty, then the jiffies update is still serialized by
          * jiffies_lock.
          */
-        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
+        if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)
+            && !tick_nohz_full_cpu(cpu))
                 tick_do_timer_cpu = cpu;
 #endif
 
@@ -123,7 +128,7 @@ static void tick_sched_do_timer(ktime_t now)
 
 static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
 {
-#ifdef CONFIG_NO_HZ
+#ifdef CONFIG_NO_HZ_COMMON
         /*
          * When we are idle and the tick is stopped, we have to touch
          * the watchdog as we might not schedule for a really long
@@ -142,10 +147,226 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
         profile_tick(CPU_PROFILING);
 }
 
+#ifdef CONFIG_NO_HZ_FULL
+static cpumask_var_t nohz_full_mask;
+bool have_nohz_full_mask;
+
+static bool can_stop_full_tick(void)
+{
+        WARN_ON_ONCE(!irqs_disabled());
+
+        if (!sched_can_stop_tick()) {
+                trace_tick_stop(0, "more than 1 task in runqueue\n");
+                return false;
+        }
+
+        if (!posix_cpu_timers_can_stop_tick(current)) {
+                trace_tick_stop(0, "posix timers running\n");
+                return false;
+        }
+
+        if (!perf_event_can_stop_tick()) {
+                trace_tick_stop(0, "perf events running\n");
+                return false;
+        }
+
+        /* sched_clock_tick() needs us? */
+#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+        /*
+         * TODO: kick full dynticks CPUs when
+         * sched_clock_stable is set.
+         */
+        if (!sched_clock_stable) {
+                trace_tick_stop(0, "unstable sched clock\n");
+                return false;
+        }
+#endif
+
+        return true;
+}
+
+static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now);
+
+/*
+ * Re-evaluate the need for the tick on the current CPU
+ * and restart it if necessary.
+ */
+void tick_nohz_full_check(void)
+{
+        struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
+
+        if (tick_nohz_full_cpu(smp_processor_id())) {
+                if (ts->tick_stopped && !is_idle_task(current)) {
+                        if (!can_stop_full_tick())
+                                tick_nohz_restart_sched_tick(ts, ktime_get());
+                }
+        }
+}
+
+static void nohz_full_kick_work_func(struct irq_work *work)
+{
+        tick_nohz_full_check();
+}
+
+static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) = {
+        .func = nohz_full_kick_work_func,
+};
+
+/*
+ * Kick the current CPU if it's full dynticks in order to force it to
+ * re-evaluate its dependency on the tick and restart it if necessary.
+ */
+void tick_nohz_full_kick(void)
+{
+        if (tick_nohz_full_cpu(smp_processor_id()))
+                irq_work_queue(&__get_cpu_var(nohz_full_kick_work));
+}
+
+static void nohz_full_kick_ipi(void *info)
+{
+        tick_nohz_full_check();
+}
+
+/*
+ * Kick all full dynticks CPUs in order to force these to re-evaluate
+ * their dependency on the tick and restart it if necessary.
+ */
+void tick_nohz_full_kick_all(void)
+{
+        if (!have_nohz_full_mask)
+                return;
+
+        preempt_disable();
+        smp_call_function_many(nohz_full_mask,
+                               nohz_full_kick_ipi, NULL, false);
+        preempt_enable();
+}
+
+/*
+ * Re-evaluate the need for the tick as we switch the current task.
+ * It might need the tick due to per task/process properties:
+ * perf events, posix cpu timers, ...
+ */
+void tick_nohz_task_switch(struct task_struct *tsk)
+{
+        unsigned long flags;
+
+        local_irq_save(flags);
+
+        if (!tick_nohz_full_cpu(smp_processor_id()))
+                goto out;
+
+        if (tick_nohz_tick_stopped() && !can_stop_full_tick())
+                tick_nohz_full_kick();
+
+out:
+        local_irq_restore(flags);
+}
+
+int tick_nohz_full_cpu(int cpu)
+{
+        if (!have_nohz_full_mask)
+                return 0;
+
+        return cpumask_test_cpu(cpu, nohz_full_mask);
+}
+
+/* Parse the boot-time nohz CPU list from the kernel parameters. */
+static int __init tick_nohz_full_setup(char *str)
+{
+        int cpu;
+
+        alloc_bootmem_cpumask_var(&nohz_full_mask);
+        if (cpulist_parse(str, nohz_full_mask) < 0) {
+                pr_warning("NOHZ: Incorrect nohz_full cpumask\n");
+                return 1;
+        }
+
+        cpu = smp_processor_id();
+        if (cpumask_test_cpu(cpu, nohz_full_mask)) {
+                pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
+                cpumask_clear_cpu(cpu, nohz_full_mask);
+        }
+        have_nohz_full_mask = true;
+
+        return 1;
+}
+__setup("nohz_full=", tick_nohz_full_setup);
+
+static int __cpuinit tick_nohz_cpu_down_callback(struct notifier_block *nfb,
+                                                 unsigned long action,
+                                                 void *hcpu)
+{
+        unsigned int cpu = (unsigned long)hcpu;
+
+        switch (action & ~CPU_TASKS_FROZEN) {
+        case CPU_DOWN_PREPARE:
+                /*
+                 * If we handle the timekeeping duty for full dynticks CPUs,
+                 * we can't safely shutdown that CPU.
+                 */
+                if (have_nohz_full_mask && tick_do_timer_cpu == cpu)
+                        return -EINVAL;
+                break;
+        }
+        return NOTIFY_OK;
+}
+
+/*
+ * Worst case string length in chunks of CPU range seems 2 steps
+ * separations: 0,2,4,6,...
+ * This is NR_CPUS + sizeof('\0')
+ */
+static char __initdata nohz_full_buf[NR_CPUS + 1];
+
+static int tick_nohz_init_all(void)
+{
+        int err = -1;
+
+#ifdef CONFIG_NO_HZ_FULL_ALL
+        if (!alloc_cpumask_var(&nohz_full_mask, GFP_KERNEL)) {
+                pr_err("NO_HZ: Can't allocate full dynticks cpumask\n");
+                return err;
+        }
+        err = 0;
+        cpumask_setall(nohz_full_mask);
+        cpumask_clear_cpu(smp_processor_id(), nohz_full_mask);
+        have_nohz_full_mask = true;
+#endif
+        return err;
+}
+
+void __init tick_nohz_init(void)
+{
+        int cpu;
+
+        if (!have_nohz_full_mask) {
+                if (tick_nohz_init_all() < 0)
+                        return;
+        }
+
+        cpu_notifier(tick_nohz_cpu_down_callback, 0);
+
+        /* Make sure full dynticks CPU are also RCU nocbs */
+        for_each_cpu(cpu, nohz_full_mask) {
+                if (!rcu_is_nocb_cpu(cpu)) {
+                        pr_warning("NO_HZ: CPU %d is not RCU nocb: "
+                                   "cleared from nohz_full range", cpu);
+                        cpumask_clear_cpu(cpu, nohz_full_mask);
+                }
+        }
+
+        cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask);
+        pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);
+}
+#else
+#define have_nohz_full_mask (0)
+#endif
+
 /*
  * NOHZ - aka dynamic tick functionality
  */
-#ifdef CONFIG_NO_HZ
+#ifdef CONFIG_NO_HZ_COMMON
 /*
  * NO HZ enabled ?
  */
@@ -345,11 +566,12 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
                         delta_jiffies = rcu_delta_jiffies;
                 }
         }
+
         /*
-         * Do not stop the tick, if we are only one off
-         * or if the cpu is required for rcu
+         * Do not stop the tick, if we are only one off (or less)
+         * or if the cpu is required for RCU:
          */
-        if (!ts->tick_stopped && delta_jiffies == 1)
+        if (!ts->tick_stopped && delta_jiffies <= 1)
                 goto out;
 
         /* Schedule the tick, if we are at least one jiffie off */
@@ -378,6 +600,13 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
                         time_delta = KTIME_MAX;
                 }
 
+#ifdef CONFIG_NO_HZ_FULL
+                if (!ts->inidle) {
+                        time_delta = min(time_delta,
+                                         scheduler_tick_max_deferment());
+                }
+#endif
+
                 /*
                  * calculate the expiry time for the next timer wheel
                  * timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals
@@ -421,6 +650,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 
                         ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
                         ts->tick_stopped = 1;
+                        trace_tick_stop(1, " ");
                 }
 
                 /*
@@ -457,6 +687,24 @@ out:
         return ret;
 }
 
+static void tick_nohz_full_stop_tick(struct tick_sched *ts)
+{
+#ifdef CONFIG_NO_HZ_FULL
+       int cpu = smp_processor_id();
+
+       if (!tick_nohz_full_cpu(cpu) || is_idle_task(current))
+               return;
+
+       if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
+               return;
+
+       if (!can_stop_full_tick())
+               return;
+
+       tick_nohz_stop_sched_tick(ts, ktime_get(), cpu);
+#endif
+}
+
 static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 {
         /*
@@ -489,6 +737,21 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
                 return false;
         }
 
+        if (have_nohz_full_mask) {
+                /*
+                 * Keep the tick alive to guarantee timekeeping progression
+                 * if there are full dynticks CPUs around
+                 */
+                if (tick_do_timer_cpu == cpu)
+                        return false;
+                /*
+                 * Boot safety: make sure the timekeeping duty has been
+                 * assigned before entering dyntick-idle mode,
+                 */
+                if (tick_do_timer_cpu == TICK_DO_TIMER_NONE)
+                        return false;
+        }
+
         return true;
 }
 
@@ -568,12 +831,13 @@ void tick_nohz_irq_exit(void)
 {
         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 
-        if (!ts->inidle)
-                return;
-
-        /* Cancel the timer because CPU already waken up from the C-states*/
-        menu_hrtimer_cancel();
-        __tick_nohz_idle_enter(ts);
+        if (ts->inidle) {
+                /* Cancel the timer because CPU already waken up from the C-states*/
+                menu_hrtimer_cancel();
+                __tick_nohz_idle_enter(ts);
+        } else {
+                tick_nohz_full_stop_tick(ts);
+        }
 }
 
 /**
@@ -802,7 +1066,7 @@ static inline void tick_check_nohz(int cpu)
 static inline void tick_nohz_switch_to_nohz(void) { }
 static inline void tick_check_nohz(int cpu) { }
 
-#endif /* NO_HZ */
+#endif /* CONFIG_NO_HZ_COMMON */
 
 /*
  * Called from irq_enter to notify about the possible interruption of idle()
@@ -887,14 +1151,14 @@ void tick_setup_sched_timer(void)
                 now = ktime_get();
         }
 
-#ifdef CONFIG_NO_HZ
+#ifdef CONFIG_NO_HZ_COMMON
         if (tick_nohz_enabled)
                 ts->nohz_mode = NOHZ_MODE_HIGHRES;
 #endif
 }
 #endif /* HIGH_RES_TIMERS */
 
-#if defined CONFIG_NO_HZ || defined CONFIG_HIGH_RES_TIMERS
+#if defined CONFIG_NO_HZ_COMMON || defined CONFIG_HIGH_RES_TIMERS
 void tick_cancel_sched_timer(int cpu)
 {
         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);