Re: [PATCH -rt] catch put_task_struct RCU handling up to mainline

[Esben Nielsen <nielsen.esben@googlemail.com>]

On Fri, 7 Jul 2006, Paul E. McKenney wrote:

> Hello!
>
> Due to the separate -rt and mainline evolution of RCU signal handling,
> the -rt patchset now makes each task struct go through two RCU grace
> periods, with one call_rcu() in release_task() and with another
> in put_task_struct().  Only the call_rcu() in release_task() is
> required, since this is the one that is associated with tearing down
> the task structure.
>
> This patch removes the extra call_rcu() in put_task_struct(), synching
> this up with mainline.  Tested lightly on i386.
>

The extra call_rcu() has an advantage:
It defers work away from the task doing the last put_task_struct().
It could be a priority 99 task with hard latency requirements doing 
some PI boosting, forinstance. The extra call_rcu() defers non-RT work to 
a low priority task. This is in generally a very good idea in a real-time 
system.
So unless you can argue that the work defered is as small as the work of 
doing a call_rcu() I would prefer the extra call_rcu().

Esben

> CC: Oleg Nesterov <oleg@tv-sign.ru>
> Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
> ---
>
> include/linux/sched.h |   10 ----------
> kernel/fork.c         |   21 ---------------------
> 2 files changed, 31 deletions(-)
>
> diff -urpNa -X dontdiff linux-2.6.17-rt5/include/linux/sched.h linux-2.6.17-rt5-2RCU/include/linux/sched.h
> --- linux-2.6.17-rt5/include/linux/sched.h	2006-07-02 12:37:14.000000000 -0700
> +++ linux-2.6.17-rt5-2RCU/include/linux/sched.h	2006-07-06 18:11:49.000000000 -0700
> @@ -1105,15 +1105,6 @@ static inline int pid_alive(struct task_
> extern void free_task(struct task_struct *tsk);
> #define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0)
>
> -#ifdef CONFIG_PREEMPT_RT
> -extern void __put_task_struct_cb(struct rcu_head *rhp);
> -
> -static inline void put_task_struct(struct task_struct *t)
> -{
> -	if (atomic_dec_and_test(&t->usage))
> -		call_rcu(&t->rcu, __put_task_struct_cb);
> -}
> -#else
> extern void __put_task_struct(struct task_struct *t);
>
> static inline void put_task_struct(struct task_struct *t)
> @@ -1121,7 +1112,6 @@ static inline void put_task_struct(struc
> 	if (atomic_dec_and_test(&t->usage))
> 		__put_task_struct(t);
> }
> -#endif
>
> /*
>  * Per process flags
> diff -urpNa -X dontdiff linux-2.6.17-rt5/kernel/fork.c linux-2.6.17-rt5-2RCU/kernel/fork.c
> --- linux-2.6.17-rt5/kernel/fork.c	2006-07-02 12:37:15.000000000 -0700
> +++ linux-2.6.17-rt5-2RCU/kernel/fork.c	2006-07-07 07:44:36.000000000 -0700
> @@ -120,26 +120,6 @@ void free_task(struct task_struct *tsk)
> }
> EXPORT_SYMBOL(free_task);
>
> -#ifdef CONFIG_PREEMPT_RT
> -void __put_task_struct_cb(struct rcu_head *rhp)
> -{
> -	struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);
> -
> -	BUG_ON(atomic_read(&tsk->usage));
> -	WARN_ON(!(tsk->flags & PF_DEAD));
> -	WARN_ON(!(tsk->exit_state & (EXIT_DEAD | EXIT_ZOMBIE)));
> -	WARN_ON(tsk == current);
> -
> -	security_task_free(tsk);
> -	free_uid(tsk->user);
> -	put_group_info(tsk->group_info);
> -
> -	if (!profile_handoff_task(tsk))
> -		free_task(tsk);
> -}
> -
> -#else
> -
> void __put_task_struct(struct task_struct *tsk)
> {
> 	WARN_ON(!(tsk->exit_state & (EXIT_DEAD | EXIT_ZOMBIE)));
> @@ -154,7 +134,6 @@ void __put_task_struct(struct task_struc
> 	if (!profile_handoff_task(tsk))
> 		free_task(tsk);
> }
> -#endif
>
> void __init fork_init(unsigned long mempages)
> {
> -
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