Re: [PATCH -tip] Create rcutree plugins to handle hotplug CPU for multi-level trees

Josh Triplett
2009-08-25T18:37:53Z

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