Re: [PATCH -rt 5/5] cpu-hotplug: cpu_down vs preempt-rt

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 10, 2008 at 01:13:04PM +0200, Peter Zijlstra wrote:
> idle_task_exit() calls mmdrop() from the idle thread, but in PREEMPT_RT all the
> allocator locks are sleeping locks - for obvious reasons scheduling away the
> idle thread gives some curious problems.
> 
> Solve this by pushing the mmdrop() into an RCU callback, however we can't use
> RCU because the CPU is already down and all the local RCU state has been
> destroyed.
> 
> Therefore create a new call_rcu() variant that enqueues the callback on an
> online cpu.

I am a bit nervous about the non-determinism, but on the other hand
CPU online/offline events can only happen so often due to the locking.

So...

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  include/linux/mm_types.h   |    5 +++++
>  include/linux/rcupreempt.h |    2 ++
>  kernel/rcupreempt.c        |   29 +++++++++++++++++++++++++++++
>  kernel/sched.c             |   13 +++++++++++++
>  4 files changed, 49 insertions(+)
> 
> Index: linux-2.6.24.7.noarch/include/linux/mm_types.h
> ===================================================================
> --- linux-2.6.24.7.noarch.orig/include/linux/mm_types.h
> +++ linux-2.6.24.7.noarch/include/linux/mm_types.h
> @@ -10,6 +10,7 @@
>  #include <linux/rbtree.h>
>  #include <linux/rwsem.h>
>  #include <linux/completion.h>
> +#include <linux/rcupdate.h>
>  #include <asm/page.h>
>  #include <asm/mmu.h>
> 
> @@ -222,6 +223,10 @@ struct mm_struct {
>  	/* aio bits */
>  	rwlock_t		ioctx_list_lock;
>  	struct kioctx		*ioctx_list;
> +
> +#ifdef CONFIG_PREEMPT_RT
> +	struct rcu_head rcu_head;
> +#endif
>  };
> 
>  #endif /* _LINUX_MM_TYPES_H */
> Index: linux-2.6.24.7.noarch/include/linux/rcupreempt.h
> ===================================================================
> --- linux-2.6.24.7.noarch.orig/include/linux/rcupreempt.h
> +++ linux-2.6.24.7.noarch/include/linux/rcupreempt.h
> @@ -83,6 +83,8 @@ extern void FASTCALL(call_rcu_classic(st
>  		     void (*func)(struct rcu_head *head)));
>  extern void FASTCALL(call_rcu_preempt(struct rcu_head *head,
>  		     void (*func)(struct rcu_head *head)));
> +extern void FASTCALL(call_rcu_preempt_online(struct rcu_head *head,
> +		     void (*func)(struct rcu_head *head)));
>  extern void __rcu_read_lock(void);
>  extern void __rcu_read_unlock(void);
>  extern void __synchronize_sched(void);
> Index: linux-2.6.24.7.noarch/kernel/rcupreempt.c
> ===================================================================
> --- linux-2.6.24.7.noarch.orig/kernel/rcupreempt.c
> +++ linux-2.6.24.7.noarch/kernel/rcupreempt.c
> @@ -916,6 +916,35 @@ void fastcall call_rcu_preempt(struct rc
>  }
>  EXPORT_SYMBOL_GPL(call_rcu_preempt);
> 
> +void fastcall call_rcu_preempt_online(struct rcu_head *head,
> +		void (*func)(struct rcu_head *rcu))
> +{
> +	struct rcu_data *rdp;
> +	unsigned long flags;
> +	int cpu;
> +
> +	head->func = func;
> +	head->next = NULL;
> +again:
> +	cpu = first_cpu(cpu_online_map);
> +	rdp = RCU_DATA_CPU(cpu);
> +
> +	spin_lock_irqsave(&rdp->lock, flags);
> +	if (unlikely(!cpu_online(cpu))) {
> +		/*
> +		 * cpu is removed from the online map before rcu_offline_cpu
> +		 * is called.
> +		 */
> +		spin_unlock_irqrestore(&rdp->lock, flags);
> +		goto again;
> +	}
> +
> +	*rdp->nexttail = head;
> +	rdp->nexttail = &head->next;
> +	spin_unlock_irqrestore(&rdp->lock, flags);
> +
> +}
> +
>  /*
>   * Check to see if any future RCU-related work will need to be done
>   * by the current CPU, even if none need be done immediately, returning
> Index: linux-2.6.24.7.noarch/kernel/sched.c
> ===================================================================
> --- linux-2.6.24.7.noarch.orig/kernel/sched.c
> +++ linux-2.6.24.7.noarch/kernel/sched.c
> @@ -5888,6 +5888,15 @@ void sched_idle_next(void)
>  	spin_unlock_irqrestore(&rq->lock, flags);
>  }
> 
> +#ifdef CONFIG_PREEMPT_RT
> +void mmdrop_rcu(struct rcu_head *head)
> +{
> +	struct mm_struct *mm = container_of(head, struct mm_struct, rcu_head);
> +
> +	mmdrop(mm);
> +}
> +#endif
> +
>  /*
>   * Ensures that the idle task is using init_mm right before its cpu goes
>   * offline.
> @@ -5900,7 +5909,11 @@ void idle_task_exit(void)
> 
>  	if (mm != &init_mm)
>  		switch_mm(mm, &init_mm, current);
> +#ifdef CONFIG_PREEMPT_RT
> +	call_rcu_preempt_online(&mm->rcu_head, mmdrop_rcu);
> +#else
>  	mmdrop(mm);
> +#endif
>  }
> 
>  /* called under rq->lock with disabled interrupts */
> 
> -- 
>