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

Avi Kivity
2011-07-21T15:37:11Z

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);
> +}
> +EXPORT_SYMBOL_GPL(preempt_notifier_register_task);
> +
> +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);
> +}
> +EXPORT_SYMBOL_GPL(preempt_notifier_unregister_task);
> +
> /**
> * 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);
> }
> EXPORT_SYMBOL_GPL(preempt_notifier_register);

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

--
error compiling committee.c: too many arguments to function