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

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Date:	Mon, 16 Nov 2009 20:31:18 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	andi@...stfloor.org, fweisbec@...il.com
Subject: Re: [PATCH 04/21] sched: implement scheduler notifiers

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.

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