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Date: Tue, 10 Jun 2008 08:33:01 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Steven Rostedt <rostedt@...dmis.org>,
Clark Williams <williams@...hat.com>,
Gregory Haskins <ghaskins@...ell.com>,
Gautham R Shenoy <ego@...ibm.com>,
Pekka Enberg <penberg@...helsinki.fi>,
Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH -rt 5/5] cpu-hotplug: cpu_down vs preempt-rt
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@...ux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.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 */
>
> --
>
--
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