sched/nohz: Clean up nohz enter/exit

The primary observation is that nohz enter/exit is always from the
current CPU, therefore NOHZ_TICK_STOPPED does not in fact need to be
an atomic.

Secondary is that we appear to have 2 nearly identical hooks in the
nohz enter code, set_cpu_sd_state_idle() and
nohz_balance_enter_idle(). Fold the whole set_cpu_sd_state thing into
nohz_balance_{enter,exit}_idle.

Removes an atomic op from both enter and exit paths.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/sched/nohz.h b/include/linux/sched/nohz.h
index 0942172..b36f4cf 100644 (file)
--- a/include/linux/sched/nohz.h
+++ b/include/linux/sched/nohz.h
@@ -16,11 +16,9 @@ static inline void cpu_load_update_nohz_stop(void) { }
 
 #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
 extern void nohz_balance_enter_idle(int cpu);
-extern void set_cpu_sd_state_idle(void);
 extern int get_nohz_timer_target(void);
 #else
 static inline void nohz_balance_enter_idle(int cpu) { }
-static inline void set_cpu_sd_state_idle(void) { }
 #endif
 
 #ifdef CONFIG_NO_HZ_COMMON

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8a10a2c..c7faeb7 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5861,7 +5861,7 @@ int sched_cpu_dying(unsigned int cpu)
 
        calc_load_migrate(rq);
        update_max_interval();
-       nohz_balance_exit_idle(cpu);
+       nohz_balance_exit_idle(rq);
        hrtick_clear(rq);
        return 0;
 }

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 85232da..494d5db 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9103,23 +9103,6 @@ static inline int find_new_ilb(void)
        return nr_cpu_ids;
 }
 
-static inline void set_cpu_sd_state_busy(void)
-{
-       struct sched_domain *sd;
-       int cpu = smp_processor_id();
-
-       rcu_read_lock();
-       sd = rcu_dereference(per_cpu(sd_llc, cpu));
-
-       if (!sd || !sd->nohz_idle)
-               goto unlock;
-       sd->nohz_idle = 0;
-
-       atomic_inc(&sd->shared->nr_busy_cpus);
-unlock:
-       rcu_read_unlock();
-}
-
 /*
  * Kick a CPU to do the nohz balancing, if it is time for it. We pick the
  * nohz_load_balancer CPU (if there is one) otherwise fallback to any idle
@@ -9175,8 +9158,7 @@ static void nohz_balancer_kick(struct rq *rq)
         * We may be recently in ticked or tickless idle mode. At the first
         * busy tick after returning from idle, we will update the busy stats.
         */
-       set_cpu_sd_state_busy();
-       nohz_balance_exit_idle(cpu);
+       nohz_balance_exit_idle(rq);
 
        /*
         * None are in tickless mode and hence no need for NOHZ idle load
@@ -9240,27 +9222,39 @@ out:
                kick_ilb(flags);
 }
 
-void nohz_balance_exit_idle(unsigned int cpu)
+static void set_cpu_sd_state_busy(int cpu)
 {
-       unsigned int flags = atomic_read(nohz_flags(cpu));
+       struct sched_domain *sd;
 
-       if (unlikely(flags & NOHZ_TICK_STOPPED)) {
-               /*
-                * Completely isolated CPUs don't ever set, so we must test.
-                */
-               if (likely(cpumask_test_cpu(cpu, nohz.idle_cpus_mask))) {
-                       cpumask_clear_cpu(cpu, nohz.idle_cpus_mask);
-                       atomic_dec(&nohz.nr_cpus);
-               }
+       rcu_read_lock();
+       sd = rcu_dereference(per_cpu(sd_llc, cpu));
 
-               atomic_andnot(NOHZ_TICK_STOPPED, nohz_flags(cpu));
-       }
+       if (!sd || !sd->nohz_idle)
+               goto unlock;
+       sd->nohz_idle = 0;
+
+       atomic_inc(&sd->shared->nr_busy_cpus);
+unlock:
+       rcu_read_unlock();
 }
 
-void set_cpu_sd_state_idle(void)
+void nohz_balance_exit_idle(struct rq *rq)
+{
+       SCHED_WARN_ON(rq != this_rq());
+
+       if (likely(!rq->nohz_tick_stopped))
+               return;
+
+       rq->nohz_tick_stopped = 0;
+       cpumask_clear_cpu(rq->cpu, nohz.idle_cpus_mask);
+       atomic_dec(&nohz.nr_cpus);
+
+       set_cpu_sd_state_busy(rq->cpu);
+}
+
+static void set_cpu_sd_state_idle(int cpu)
 {
        struct sched_domain *sd;
-       int cpu = smp_processor_id();
 
        rcu_read_lock();
        sd = rcu_dereference(per_cpu(sd_llc, cpu));
@@ -9280,6 +9274,10 @@ unlock:
  */
 void nohz_balance_enter_idle(int cpu)
 {
+       struct rq *rq = cpu_rq(cpu);
+
+       SCHED_WARN_ON(cpu != smp_processor_id());
+
        /* If this CPU is going down, then nothing needs to be done: */
        if (!cpu_active(cpu))
                return;
@@ -9288,16 +9286,19 @@ void nohz_balance_enter_idle(int cpu)
        if (!housekeeping_cpu(cpu, HK_FLAG_SCHED))
                return;
 
-       if (atomic_read(nohz_flags(cpu)) & NOHZ_TICK_STOPPED)
+       if (rq->nohz_tick_stopped)
                return;
 
        /* If we're a completely isolated CPU, we don't play: */
-       if (on_null_domain(cpu_rq(cpu)))
+       if (on_null_domain(rq))
                return;
 
+       rq->nohz_tick_stopped = 1;
+
        cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
        atomic_inc(&nohz.nr_cpus);
-       atomic_or(NOHZ_TICK_STOPPED, nohz_flags(cpu));
+
+       set_cpu_sd_state_idle(cpu);
 }
 #else
 static inline void nohz_balancer_kick(struct rq *rq) { }

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 21381d2..818f22d 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -764,6 +764,7 @@ struct rq {
        unsigned long           last_load_update_tick;
        unsigned long           last_blocked_load_update_tick;
 #endif /* CONFIG_SMP */
+       unsigned int            nohz_tick_stopped;
        atomic_t nohz_flags;
 #endif /* CONFIG_NO_HZ_COMMON */
 
@@ -2035,11 +2036,9 @@ extern void cfs_bandwidth_usage_inc(void);
 extern void cfs_bandwidth_usage_dec(void);
 
 #ifdef CONFIG_NO_HZ_COMMON
-#define NOHZ_TICK_STOPPED_BIT  0
-#define NOHZ_BALANCE_KICK_BIT  1
-#define NOHZ_STATS_KICK_BIT    2
+#define NOHZ_BALANCE_KICK_BIT  0
+#define NOHZ_STATS_KICK_BIT    1
 
-#define NOHZ_TICK_STOPPED      BIT(NOHZ_TICK_STOPPED_BIT)
 #define NOHZ_BALANCE_KICK      BIT(NOHZ_BALANCE_KICK_BIT)
 #define NOHZ_STATS_KICK                BIT(NOHZ_STATS_KICK_BIT)
 
@@ -2047,9 +2046,9 @@ extern void cfs_bandwidth_usage_dec(void);
 
 #define nohz_flags(cpu)        (&cpu_rq(cpu)->nohz_flags)
 
-extern void nohz_balance_exit_idle(unsigned int cpu);
+extern void nohz_balance_exit_idle(struct rq *rq);
 #else
-static inline void nohz_balance_exit_idle(unsigned int cpu) { }
+static inline void nohz_balance_exit_idle(struct rq *rq) { }
 #endif
 
 

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f2fa2e9..ab92aa4 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -954,13 +954,6 @@ void tick_nohz_idle_enter(void)
        struct tick_sched *ts;
 
        lockdep_assert_irqs_enabled();
-       /*
-        * Update the idle state in the scheduler domain hierarchy
-        * when tick_nohz_stop_sched_tick() is called from the idle loop.
-        * State will be updated to busy during the first busy tick after
-        * exiting idle.
-        */
-       set_cpu_sd_state_idle();
 
        local_irq_disable();