Commit-ID: b3e0b1b6d841a4b2f64fc09ea728913da8218424
Gitweb: http://git.kernel.org/tip/b3e0b1b6d841a4b2f64fc09ea728913da8218424
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Fri, 16 Oct 2015 14:39:38 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 4 Dec 2015 10:33:41 +0100
locking, sched: Introduce smp_cond_acquire() and use it
Introduce smp_cond_acquire() which combines a control dependency and a
read barrier to form acquire semantics.
This primitive has two benefits:
- it documents control dependencies,
- its typically cheaper than using smp_load_acquire() in a loop.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
include/linux/compiler.h | 17 +++++++++++++++++
kernel/locking/qspinlock.c | 3 +--
kernel/sched/core.c | 8 +-------
kernel/sched/sched.h | 2 +-
4 files changed, 20 insertions(+), 10 deletions(-)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4dac103..00b042c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -299,6 +299,23 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
__u.__val; \
})
+/**
+ * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * @cond: boolean expression to wait for
+ *
+ * Equivalent to using smp_load_acquire() on the condition variable but employs
+ * the control dependency of the wait to reduce the barrier on many platforms.
+ *
+ * The control dependency provides a LOAD->STORE order, the additional RMB
+ * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
+ * aka. ACQUIRE.
+ */
+#define smp_cond_acquire(cond) do { \
+ while (!(cond)) \
+ cpu_relax(); \
+ smp_rmb(); /* ctrl + rmb := acquire */ \
+} while (0)
+
#endif /* __KERNEL__ */
#endif /* __ASSEMBLY__ */
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 9862078..ed9d967 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -433,8 +433,7 @@ queue:
*
*/
pv_wait_head(lock, node);
- while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
- cpu_relax();
+ smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
/*
* claim the lock:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7063c6a..9f7862d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1968,19 +1968,13 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
/*
* If the owning (remote) cpu is still in the middle of schedule() with
* this task as prev, wait until its done referencing the task.
- */
- while (p->on_cpu)
- cpu_relax();
- /*
- * Combined with the control dependency above, we have an effective
- * smp_load_acquire() without the need for full barriers.
*
* Pairs with the smp_store_release() in finish_lock_switch().
*
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
*/
- smp_rmb();
+ smp_cond_acquire(!p->on_cpu);
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b242775..1e0bb4af 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1076,7 +1076,7 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
* In particular, the load of prev->state in finish_task_switch() must
* happen before this.
*
- * Pairs with the control dependency and rmb in try_to_wake_up().
+ * Pairs with the smp_cond_acquire() in try_to_wake_up().
*/
smp_store_release(&prev->on_cpu, 0);
#endif
--
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