Re: [PATCH 1/3] perf: add context field to perf_event

From: Avi Kivity
Date: Thu Jul 21 2011 - 11:37:15 EST

On 07/21/2011 06:32 PM, Will Deacon wrote:
> Using TIF_bits sounds like a much better solution for this, wakeups are
> really rather expensive and its best to avoid extra if at all possible.

The problem with using a TIF bit to tell a task that it needs to perform
some preempt_notifier registrations is that you end up with something that
looks a lot like preempt notifiers! You also don't escape the concurrent
read/write to thelist of pending registrations.

One thing I tried was simply using an RCU protected hlist for the preempt
notifiers so that we don't have to worry about atomicity when reading the
notifiers in finish_task_switch. It's a bit odd, since we know we only ever
have a single reader, but I've included it below anyway.

If anybody has any better ideas, I'm all ears.

+void preempt_notifier_register_task(struct preempt_notifier *notifier,
+ struct task_struct *tsk)
+ mutex_lock(&tsk->preempt_notifiers_mutex);
+ hlist_add_head_rcu(&notifier->link,&tsk->preempt_notifiers);
+ mutex_unlock(&tsk->preempt_notifiers_mutex);
+void preempt_notifier_unregister_task(struct preempt_notifier *notifier,
+ struct task_struct *tsk)
+ mutex_lock(&tsk->preempt_notifiers_mutex);
+ hlist_del_rcu(&notifier->link);
+ mutex_unlock(&tsk->preempt_notifiers_mutex);
* preempt_notifier_register - tell me when current is being preempted& rescheduled
* @notifier: notifier struct to register
void preempt_notifier_register(struct preempt_notifier *notifier)
- hlist_add_head(&notifier->link,&current->preempt_notifiers);
+ preempt_notifier_register_task(notifier, current);

This is (and must be) called from a preempt disabled context, no mutexes around here.

error compiling committee.c: too many arguments to function

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