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rcu: Create rcutree plugins to handle hotplug CPU for multi-level trees 

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.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <20090827215816.GA30472@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Paul E. McKenney 2009-08-27 14:58:16 -07:00 committed by Ingo Molnar
parent 06e799764e
commit dd5d19bafd
4 changed files with 69 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
include/linux/init_task.h
View File

@ -98,7 +98,7 @@ extern struct group_info init_groups;
#define INIT_TASK_RCU_PREEMPT(tsk) \
.rcu_read_lock_nesting = 0, \
.rcu_read_unlock_special = 0, \
.rcu_blocked_cpu = -1, \
.rcu_blocked_node = NULL, \
.rcu_node_entry = LIST_HEAD_INIT(tsk.rcu_node_entry),
#else
#define INIT_TASK_RCU_PREEMPT(tsk)

4
include/linux/sched.h
View File

@ -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;
struct list_head rcu_node_entry;
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
@ -1735,7 +1735,7 @@ static inline void rcu_copy_process(struct task_struct *p)
{
p->rcu_read_lock_nesting = 0;
p->rcu_read_unlock_special = 0;
p->rcu_blocked_cpu = -1;
p->rcu_blocked_node = NULL;
INIT_LIST_HEAD(&p->rcu_node_entry);
}

2
kernel/rcutree.c
View File

@ -81,6 +81,7 @@ struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
extern long rcu_batches_completed_sched(void);
static struct rcu_node *rcu_get_root(struct rcu_state *rsp);
static void cpu_quiet_msk(unsigned long mask, struct rcu_state *rsp,
struct rcu_node *rnp, unsigned long flags);
static void cpu_quiet_msk_finish(struct rcu_state *rsp, unsigned long flags);
@ -876,6 +877,7 @@ static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
spin_unlock(&rnp->lock); /* irqs remain disabled. */
break;
}
rcu_preempt_offline_tasks(rsp, rnp);
mask = rnp->grpmask;
spin_unlock(&rnp->lock); /* irqs remain disabled. */
rnp = rnp->parent;

69
kernel/rcutree_plugin.h
View File

@ -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;
/*
* If this CPU has already checked in, then this task
@ -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);
}
empty = list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1]);
list_del_init(&t->rcu_node_entry);
t->rcu_blocked_cpu = -1;
t->rcu_blocked_node = NULL;
/*
* If this was the last task on the current list, and if
@ -261,6 +270,47 @@ static int rcu_preempted_readers(struct rcu_node *rnp)
#ifdef CONFIG_HOTPLUG_CPU
/*
* Handle tasklist migration for case in which all CPUs covered by the
* specified rcu_node have gone offline. Move them up to the root
* rcu_node. The reason for not just moving them to the immediate
* parent is to remove the need for rcu_read_unlock_special() to
* make more than two attempts to acquire the target rcu_node's lock.
*
* The caller must hold rnp->lock with irqs disabled.
*/
static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
struct rcu_node *rnp)
{
int i;
struct list_head *lp;
struct list_head *lp_root;
struct rcu_node *rnp_root = rcu_get_root(rsp);
struct task_struct *tp;
if (rnp == rnp_root)
return; /* Shouldn't happen: at least one CPU online. */
/*
* Move tasks up to root rcu_node. Rely on the fact that the
* root rcu_node can be at most one ahead of the rest of the
* rcu_nodes in terms of gp_num value. This fact allows us to
* move the blocked_tasks[] array directly, element by element.
*/
for (i = 0; i < 2; i++) {
lp = &rnp->blocked_tasks[i];
lp_root = &rnp_root->blocked_tasks[i];
while (!list_empty(lp)) {
tp = list_entry(lp->next, typeof(*tp), rcu_node_entry);
spin_lock(&rnp_root->lock); /* irqs already disabled */
list_del(&tp->rcu_node_entry);
tp->rcu_blocked_node = rnp_root;
list_add(&tp->rcu_node_entry, lp_root);
spin_unlock(&rnp_root->lock); /* irqs remain disabled */
}
}
}
/*
* Do CPU-offline processing for preemptable RCU.
*/
@ -409,6 +459,15 @@ static int rcu_preempted_readers(struct rcu_node *rnp)
#ifdef CONFIG_HOTPLUG_CPU
/*
* Because preemptable RCU does not exist, it never needs to migrate
* tasks that were blocked within RCU read-side critical sections.
*/
static void rcu_preempt_offline_tasks(struct rcu_state *rsp,
struct rcu_node *rnp)
{
}
/*
* Because preemptable RCU does not exist, it never needs CPU-offline
* processing.