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    SubjectRe: [PATCH -tip] Create rcutree plugins to handle hotplug CPU for multi-level trees
    On Tue, Aug 25, 2009 at 11:38:03AM -0700, Josh Triplett wrote:
    > 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.

    Good point -- it would be nice to avoid the casts.

    > > --- 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.

    Indeed. Fixed, will submit early tomorrow.

    Thanx, Paul


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