Re: [PATCH 04/21] sched: implement scheduler notifiers

Avi Kivity
2009-11-16T18:32:25Z

On 11/16/2009 07:15 PM, Tejun Heo wrote:
> Implement scheduler notifiers. This is superset of preempt notifiers
> which will be removed in favor of new notifiers. Four notifications
> are defined - activated, deactivated, in and out. In and out are
> identical to preempt notifiers. Activated and deactivated are called
> when a task's readiness to run changes. The first three are always
> called under rq lock. Out may not be called under rq lock depending
> on architecture.
>
> The notifier block contains union of all four callbacks to avoid
> defining separate interface for each.
>

> +
> +struct sched_notifier {
> + struct hlist_node link;
> + union {
> + void (*activated)(struct sched_notifier *n, bool wakeup);
> + void (*deactivated)(struct sched_notifier *n, bool sleep);
> + void (*in)(struct sched_notifier *n, struct task_struct *prev);
> + void (*out)(struct sched_notifier *n, struct task_struct *next);
> + };
> +};
> +
>
> struct task_struct {
> @@ -1237,6 +1268,8 @@ struct task_struct {
> /* list of struct preempt_notifier: */
> struct hlist_head preempt_notifiers;
> #endif
> + /* sched notifiers */
> + struct hlist_head notifiers[SCHED_NR_NOTIFIERS];
>
>

Four hlist_heads (64 bytes) is pretty heavy for this. I having all
members present in sched_notifier (instead of a union) and calling a
callback if it is not NULL. This reduces the overhead to 16 bytes at
the expense of an extra check for sched_notifier users.

Besides this, is there any difference to preempt_notifiers? if not we
can just add the new members and rename.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.