Messages in this thread | | | Subject | Re: [PATCH -tip] Create rcutree plugins to handle hotplug CPU for multi-level trees | From | Josh Triplett <> | Date | Tue, 25 Aug 2009 11:38:03 -0700 |
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On Tue, 2009-08-25 at 11:22 -0700, Paul E. McKenney wrote: > When offlining CPUs from a multi-level tree, there is the possibility > of offlining the last CPU from a given node when there are preempted > RCU read-side critical sections that started life on one of the CPUs on > that node. In this case, the corresponding tasks will be enqueued via > the task_struct's rcu_node_entry list_head onto one of the rcu_node's > blocked_tasks[] lists. These tasks need to be moved somewhere else > so that they will prevent the current grace period from ending. > That somewhere is the root rcu_node. > > With this patch, TREE_PREEMPT_RCU passes moderate rcutorture testing > with aggressive CPU-hotplugging (no delay between inserting/removing > randomly selected CPU). > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Looks good. One comment below.
> --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1208,7 +1208,7 @@ struct task_struct { > #ifdef CONFIG_TREE_PREEMPT_RCU > int rcu_read_lock_nesting; > char rcu_read_unlock_special; > - int rcu_blocked_cpu; > + void *rcu_blocked_node;
This should use struct rcu_node *, not void *. That would eliminate several casts in the changes below. You can forward-declare struct rcu_node if you want to avoid including RCU headers in sched.h.
> --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -92,7 +92,7 @@ static void rcu_preempt_qs(int cpu) > rnp = rdp->mynode; > spin_lock(&rnp->lock); > t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED; > - t->rcu_blocked_cpu = cpu; > + t->rcu_blocked_node = (void *)rnp;
Regardless of whether you change the type in the structure, you never need to cast a pointer to type void *; any non-function pointer will become void * without complaint.
> @@ -170,12 +170,21 @@ static void rcu_read_unlock_special(struct task_struct *t) > if (special & RCU_READ_UNLOCK_BLOCKED) { > t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED; > > - /* Remove this task from the list it blocked on. */ > - rnp = rcu_preempt_state.rda[t->rcu_blocked_cpu]->mynode; > - spin_lock(&rnp->lock); > + /* > + * Remove this task from the list it blocked on. The > + * task can migrate while we acquire the lock, but at > + * most one time. So at most two passes through loop. > + */ > + for (;;) { > + rnp = (struct rcu_node *)t->rcu_blocked_node; > + spin_lock(&rnp->lock); > + if (rnp == (struct rcu_node *)t->rcu_blocked_node) > + break; > + spin_unlock(&rnp->lock); > + }
Both of the casts of t->rcu_blocked_node can go away here, given the type change in the structure.
- Josh Triplett
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