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

kernel/sched/core.c

+15 −25
Original line number Diff line number Diff line
@@ -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);

}

kernel/sched/sched.h

+5 −8
Original line number Diff line number Diff line
@@ -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) \