Commit ae792702 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

sched: Optimize finish_lock_switch()

The kernel test robot measured a -1.6% performance regression on
will-it-scale/sched_yield due to commit:

  2558aacf ("sched/hotplug: Ensure only per-cpu kthreads run during hotplug")

Even though we were careful to replace a single load with another
single load from the same cacheline.

Restore finish_lock_switch() to the exact state before the offending
patch and solve the problem differently.

Fixes: 2558aacf

 ("sched/hotplug: Ensure only per-cpu kthreads run during hotplug")
Reported-by: default avatarkernel test robot <oliver.sang@intel.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201210161408.GX3021@hirez.programming.kicks-ass.net
parent 5b78f2dc
......@@ -3985,15 +3985,20 @@ static void do_balance_callbacks(struct rq *rq, struct callback_head *head)
}
}
static void balance_push(struct rq *rq);
struct callback_head balance_push_callback = {
.next = NULL,
.func = (void (*)(struct callback_head *))balance_push,
};
static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
{
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;
}
......@@ -4014,21 +4019,6 @@ static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
}
}
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)
......@@ -4044,10 +4034,6 @@ static inline void balance_callbacks(struct rq *rq, struct callback_head *head)
{
}
static inline void balance_switch(struct rq *rq)
{
}
#endif
static inline void
......@@ -4075,7 +4061,7 @@ static inline void finish_lock_switch(struct rq *rq)
* prev into current:
*/
spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
balance_switch(rq);
__balance_callbacks(rq);
raw_spin_unlock_irq(&rq->lock);
}
......@@ -7256,6 +7242,10 @@ static void balance_push(struct rq *rq)
lockdep_assert_held(&rq->lock);
SCHED_WARN_ON(rq->cpu != smp_processor_id());
/*
* Ensure the thing is persistent until balance_push_set(.on = false);
*/
rq->balance_callback = &balance_push_callback;
/*
* Both the cpu-hotplug and stop task are in this case and are
......@@ -7305,9 +7295,9 @@ static void balance_push_set(int cpu, bool on)
rq_lock_irqsave(rq, &rf);
if (on)
rq->balance_flags |= BALANCE_PUSH;
rq->balance_callback = &balance_push_callback;
else
rq->balance_flags &= ~BALANCE_PUSH;
rq->balance_callback = NULL;
rq_unlock_irqrestore(rq, &rf);
}
......
......@@ -975,7 +975,6 @@ 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;
......@@ -1226,6 +1225,8 @@ struct rq_flags {
#endif
};
extern struct callback_head balance_push_callback;
/*
* Lockdep annotation that avoids accidental unlocks; it's like a
* sticky/continuous lockdep_assert_held().
......@@ -1243,9 +1244,9 @@ static inline void rq_pin_lock(struct rq *rq, struct rq_flags *rf)
#ifdef CONFIG_SCHED_DEBUG
rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
rf->clock_update_flags = 0;
#endif
#ifdef CONFIG_SMP
SCHED_WARN_ON(rq->balance_callback);
SCHED_WARN_ON(rq->balance_callback && rq->balance_callback != &balance_push_callback);
#endif
#endif
}
......@@ -1408,9 +1409,6 @@ init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
#ifdef CONFIG_SMP
#define BALANCE_WORK 0x01
#define BALANCE_PUSH 0x02
static inline void
queue_balance_callback(struct rq *rq,
struct callback_head *head,
......@@ -1418,13 +1416,12 @@ queue_balance_callback(struct rq *rq,
{
lockdep_assert_held(&rq->lock);
if (unlikely(head->next || (rq->balance_flags & BALANCE_PUSH)))
if (unlikely(head->next || rq->balance_callback == &balance_push_callback))
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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