sched: Optimize finish_lock_switch()
The kernel test robot measured a -1.6% performance regression on
will-it-scale/sched_yield due to commit:
2558aacff858 ("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: 2558aacff858 ("sched/hotplug: Ensure only per-cpu kthreads run during hotplug")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201210161408.GX3021@hirez.programming.kicks-ass.net
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7af80c3..0ca7d2d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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);
}