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Date: Thu, 5 Feb 2015 11:53:46 +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: Re: [PATCH] sched/completion: Add fast path in __wait_for_common()
Please, ignore this patch.
This brings the problem with exiting threads like in
https://lkml.org/lkml/2015/2/4/761
В Ср, 04/02/2015 в 19:50 +0300, Kirill Tkhai пишет:
> 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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