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Message-ID: <1423068649.15721.1.camel@tkhai>
Date: Wed, 4 Feb 2015 19:50:49 +0300
From: Kirill Tkhai <ktkhai@...allels.com>
To: <linux-kernel@...r.kernel.org>
CC: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] sched/completion: Add fast path in __wait_for_common()
We may optimize __wait_for_common() in case of it's waken up
by complete{,_all}() if we do not take the spinlock.
New function completion_wake_function() is now used to wake
waiters. It a case of successful waking it deletes a waiter
from the task list. The waiter checks wait.task_list and skips
the locking if it's empty. In case of a single waiter this
prevents from unnecessary spin_{,un}_lock_irq().
In case of several waiters this improves parallelism in the
obvious way.
Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
---
include/linux/wait.h | 1 +
kernel/sched/completion.c | 57 ++++++++++++++++++++++++++++++++-------------
kernel/sched/wait.c | 7 ++++++
3 files changed, 48 insertions(+), 17 deletions(-)
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 37423e0..506adfc 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -148,6 +148,7 @@ __remove_wait_queue(wait_queue_head_t *head, wait_queue_t *old)
typedef int wait_bit_action_f(struct wait_bit_key *);
void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr, void *key);
void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key);
+void __wake_up_locked_nr_key(wait_queue_head_t *q, unsigned int mode, int nr, void *key);
void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode, int nr, void *key);
void __wake_up_locked(wait_queue_head_t *q, unsigned int mode, int nr);
void __wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr);
diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 607f852..b9959b8 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -32,7 +32,7 @@ void complete(struct completion *x)
spin_lock_irqsave(&x->wait.lock, flags);
x->done++;
- __wake_up_locked(&x->wait, TASK_NORMAL, 1);
+ __wake_up_locked_nr_key(&x->wait, TASK_NORMAL, 1, x);
spin_unlock_irqrestore(&x->wait.lock, flags);
}
EXPORT_SYMBOL(complete);
@@ -52,17 +52,36 @@ void complete_all(struct completion *x)
spin_lock_irqsave(&x->wait.lock, flags);
x->done += UINT_MAX/2;
- __wake_up_locked(&x->wait, TASK_NORMAL, 0);
+ __wake_up_locked_nr_key(&x->wait, TASK_NORMAL, 0, x);
spin_unlock_irqrestore(&x->wait.lock, flags);
}
EXPORT_SYMBOL(complete_all);
+static int completion_wake_function(wait_queue_t *wait, unsigned mode,
+ int sync, void *key)
+{
+ int ret = default_wake_function(wait, mode, sync, key);
+ struct completion *x = key;
+
+ if (ret) {
+ list_del_init(&wait->task_list);
+ x->done--;
+ }
+ return ret;
+}
+
static inline long __sched
-do_wait_for_common(struct completion *x,
- long (*action)(long), long timeout, int state)
+__wait_for_common(struct completion *x,
+ long (*action)(long), long timeout, int state)
{
+ might_sleep();
+
+ spin_lock_irq(&x->wait.lock);
if (!x->done) {
- DECLARE_WAITQUEUE(wait, current);
+ wait_queue_t wait = {
+ .private = current,
+ .func = completion_wake_function,
+ };
__add_wait_queue_tail_exclusive(&x->wait, &wait);
do {
@@ -73,26 +92,30 @@ do_wait_for_common(struct completion *x,
__set_current_state(state);
spin_unlock_irq(&x->wait.lock);
timeout = action(timeout);
+ /*
+ * This is the fast check whether we are woken up by
+ * completion_wake_function(). No spinlock held here.
+ */
+ if (list_empty(&wait.task_list))
+ goto out;
spin_lock_irq(&x->wait.lock);
+ /*
+ * The above check is unlocked and racy with the wake
+ * function. Test again to be sure, we haven't missed
+ * the sign of its work.
+ */
+ if (unlikely(list_empty(&wait.task_list)))
+ goto unlock;
} while (!x->done && timeout);
__remove_wait_queue(&x->wait, &wait);
if (!x->done)
return timeout;
}
x->done--;
- return timeout ?: 1;
-}
-
-static inline long __sched
-__wait_for_common(struct completion *x,
- long (*action)(long), long timeout, int state)
-{
- might_sleep();
-
- spin_lock_irq(&x->wait.lock);
- timeout = do_wait_for_common(x, action, timeout, state);
+unlock:
spin_unlock_irq(&x->wait.lock);
- return timeout;
+out:
+ return timeout ?: 1;
}
static long __sched
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 852143a..4846a57 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -112,6 +112,13 @@ void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key)
}
EXPORT_SYMBOL_GPL(__wake_up_locked_key);
+void __wake_up_locked_nr_key(wait_queue_head_t *q, unsigned int mode,
+ int nr, void *key)
+{
+ __wake_up_common(q, mode, nr, 0, key);
+}
+EXPORT_SYMBOL_GPL(__wake_up_locked_nr_key);
+
/**
* __wake_up_sync_key - wake up threads blocked on a waitqueue.
* @q: the waitqueue
--
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