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Message-ID: <20201023102346.292709163@infradead.org>
Date: Fri, 23 Oct 2020 12:12:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: tglx@...utronix.de, mingo@...nel.org
Cc: linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
qais.yousef@....com, swood@...hat.com, peterz@...radead.org,
valentin.schneider@....com, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, vincent.donnefort@....com, tj@...nel.org,
ouwen210@...mail.com
Subject: [PATCH v4 03/19] sched/hotplug: Ensure only per-cpu kthreads run during hotplug
In preparation for migrate_disable(), make sure only per-cpu kthreads
are allowed to run on !active CPUs.
This is ran (as one of the very first steps) from the cpu-hotplug
task which is a per-cpu kthread and completion of the hotplug
operation only requires such tasks.
This constraint enables the migrate_disable() implementation to wait
for completion of all migrate_disable regions on this CPU at hotplug
time without fear of any new ones starting.
This replaces the unlikely(rq->balance_callbacks) test at the tail of
context_switch with an unlikely(rq->balance_work), the fast path is
not affected.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
kernel/sched/core.c | 114 ++++++++++++++++++++++++++++++++++++++++++++++++++-
kernel/sched/sched.h | 7 ++-
2 files changed, 118 insertions(+), 3 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3509,8 +3509,10 @@ static inline struct callback_head *spli
struct callback_head *head = rq->balance_callback;
lockdep_assert_held(&rq->lock);
- if (head)
+ if (head) {
rq->balance_callback = NULL;
+ rq->balance_flags &= ~BALANCE_WORK;
+ }
return head;
}
@@ -3531,6 +3533,21 @@ static inline void balance_callbacks(str
}
}
+static void balance_push(struct rq *rq);
+
+static inline void balance_switch(struct rq *rq)
+{
+ if (likely(!rq->balance_flags))
+ return;
+
+ if (rq->balance_flags & BALANCE_PUSH) {
+ balance_push(rq);
+ return;
+ }
+
+ __balance_callbacks(rq);
+}
+
#else
static inline void __balance_callbacks(struct rq *rq)
@@ -3546,6 +3563,10 @@ static inline void balance_callbacks(str
{
}
+static inline void balance_switch(struct rq *rq)
+{
+}
+
#endif
static inline void
@@ -3573,7 +3594,7 @@ static inline void finish_lock_switch(st
* prev into current:
*/
spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
- __balance_callbacks(rq);
+ balance_switch(rq);
raw_spin_unlock_irq(&rq->lock);
}
@@ -6831,6 +6852,90 @@ static void migrate_tasks(struct rq *dea
rq->stop = stop;
}
+
+static int __balance_push_cpu_stop(void *arg)
+{
+ struct task_struct *p = arg;
+ struct rq *rq = this_rq();
+ struct rq_flags rf;
+ int cpu;
+
+ raw_spin_lock_irq(&p->pi_lock);
+ rq_lock(rq, &rf);
+
+ update_rq_clock(rq);
+
+ if (task_rq(p) == rq && task_on_rq_queued(p)) {
+ cpu = select_fallback_rq(rq->cpu, p);
+ rq = __migrate_task(rq, &rf, p, cpu);
+ }
+
+ rq_unlock(rq, &rf);
+ raw_spin_unlock_irq(&p->pi_lock);
+
+ put_task_struct(p);
+
+ return 0;
+}
+
+static DEFINE_PER_CPU(struct cpu_stop_work, push_work);
+
+/*
+ * Ensure we only run per-cpu kthreads once the CPU goes !active.
+ */
+static void balance_push(struct rq *rq)
+{
+ struct task_struct *push_task = rq->curr;
+
+ lockdep_assert_held(&rq->lock);
+ SCHED_WARN_ON(rq->cpu != smp_processor_id());
+
+ /*
+ * Both the cpu-hotplug and stop task are in this case and are
+ * required to complete the hotplug process.
+ */
+ if (is_per_cpu_kthread(push_task))
+ return;
+
+ get_task_struct(push_task);
+ /*
+ * Temporarily drop rq->lock such that we can wake-up the stop task.
+ * Both preemption and IRQs are still disabled.
+ */
+ raw_spin_unlock(&rq->lock);
+ stop_one_cpu_nowait(rq->cpu, __balance_push_cpu_stop, push_task,
+ this_cpu_ptr(&push_work));
+ /*
+ * At this point need_resched() is true and we'll take the loop in
+ * schedule(). The next pick is obviously going to be the stop task
+ * which is_per_cpu_kthread() and will push this task away.
+ */
+ raw_spin_lock(&rq->lock);
+}
+
+static void balance_push_set(int cpu, bool on)
+{
+ struct rq *rq = cpu_rq(cpu);
+ struct rq_flags rf;
+
+ rq_lock_irqsave(rq, &rf);
+ if (on)
+ rq->balance_flags |= BALANCE_PUSH;
+ else
+ rq->balance_flags &= ~BALANCE_PUSH;
+ rq_unlock_irqrestore(rq, &rf);
+}
+
+#else
+
+static inline void balance_push(struct rq *rq)
+{
+}
+
+static inline void balance_push_set(int cpu, bool on)
+{
+}
+
#endif /* CONFIG_HOTPLUG_CPU */
void set_rq_online(struct rq *rq)
@@ -6916,6 +7021,8 @@ int sched_cpu_activate(unsigned int cpu)
struct rq *rq = cpu_rq(cpu);
struct rq_flags rf;
+ balance_push_set(cpu, false);
+
#ifdef CONFIG_SCHED_SMT
/*
* When going up, increment the number of cores with SMT present.
@@ -6963,6 +7070,8 @@ int sched_cpu_deactivate(unsigned int cp
*/
synchronize_rcu();
+ balance_push_set(cpu, true);
+
#ifdef CONFIG_SCHED_SMT
/*
* When going down, decrement the number of cores with SMT present.
@@ -6976,6 +7085,7 @@ int sched_cpu_deactivate(unsigned int cp
ret = cpuset_cpu_inactive(cpu);
if (ret) {
+ balance_push_set(cpu, false);
set_cpu_active(cpu, true);
return ret;
}
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -973,6 +973,7 @@ struct rq {
unsigned long cpu_capacity_orig;
struct callback_head *balance_callback;
+ unsigned char balance_flags;
unsigned char nohz_idle_balance;
unsigned char idle_balance;
@@ -1385,6 +1386,9 @@ init_numa_balancing(unsigned long clone_
#ifdef CONFIG_SMP
+#define BALANCE_WORK 0x01
+#define BALANCE_PUSH 0x02
+
static inline void
queue_balance_callback(struct rq *rq,
struct callback_head *head,
@@ -1392,12 +1396,13 @@ queue_balance_callback(struct rq *rq,
{
lockdep_assert_held(&rq->lock);
- if (unlikely(head->next))
+ if (unlikely(head->next || (rq->balance_flags & BALANCE_PUSH)))
return;
head->func = (void (*)(struct callback_head *))func;
head->next = rq->balance_callback;
rq->balance_callback = head;
+ rq->balance_flags |= BALANCE_WORK;
}
#define rcu_dereference_check_sched_domain(p) \
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