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authorPaul E. McKenney <paul.mckenney@linaro.org>2012-01-30 17:02:47 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-02-21 09:06:07 -0800
commit2036d94a7b61ca5032ce90f2bda06afec0fe713e (patch)
treefc9f4b5ba1058e5a2fdf9ccd187766c90f5ae036 /kernel/rcutree.c
parentc5fdcec927ee31fc96e92339c3a83ac6e0725289 (diff)
downloadlwn-2036d94a7b61ca5032ce90f2bda06afec0fe713e.tar.gz
lwn-2036d94a7b61ca5032ce90f2bda06afec0fe713e.zip
rcu: Rework detection of use of RCU by offline CPUs
Because newly offlined CPUs continue executing after completing the CPU_DYING notifiers, they legitimately enter the scheduler and use RCU while appearing to be offline. This calls for a more sophisticated approach as follows: 1. RCU marks the CPU online during the CPU_UP_PREPARE phase. 2. RCU marks the CPU offline during the CPU_DEAD phase. 3. Diagnostics regarding use of read-side RCU by offline CPUs use RCU's accounting rather than the cpu_online_map. (Note that __call_rcu() still uses cpu_online_map to detect illegal invocations within CPU_DYING notifiers.) 4. Offline CPUs are prevented from hanging the system by force_quiescent_state(), which pays attention to cpu_online_map. Some additional work (in a later commit) will be needed to guarantee that force_quiescent_state() waits a full jiffy before assuming that a CPU is offline, for example, when called from idle entry. (This commit also makes the one-jiffy wait explicit, since the old-style implicit wait can now be defeated by RCU_FAST_NO_HZ and by rcutorture.) This approach avoids the false positives encountered when attempting to use more exact classification of CPU online/offline state. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcutree.c')
-rw-r--r--kernel/rcutree.c113
1 files changed, 67 insertions, 46 deletions
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 05470d4caba3..708469a06860 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -320,25 +320,18 @@ static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
static int rcu_implicit_offline_qs(struct rcu_data *rdp)
{
/*
- * If the CPU is offline, it is in a quiescent state. We can
- * trust its state not to change because interrupts are disabled.
+ * If the CPU is offline for more than a jiffy, it is in a quiescent
+ * state. We can trust its state not to change because interrupts
+ * are disabled. The reason for the jiffy's worth of slack is to
+ * handle CPUs initializing on the way up and finding their way
+ * to the idle loop on the way down.
*/
- if (cpu_is_offline(rdp->cpu)) {
+ if (cpu_is_offline(rdp->cpu) &&
+ ULONG_CMP_LT(rdp->rsp->gp_start + 2, jiffies)) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, "ofl");
rdp->offline_fqs++;
return 1;
}
-
- /*
- * The CPU is online, so send it a reschedule IPI. This forces
- * it through the scheduler, and (inefficiently) also handles cases
- * where idle loops fail to inform RCU about the CPU being idle.
- */
- if (rdp->cpu != smp_processor_id())
- smp_send_reschedule(rdp->cpu);
- else
- set_need_resched();
- rdp->resched_ipi++;
return 0;
}
@@ -601,19 +594,33 @@ EXPORT_SYMBOL(rcu_is_cpu_idle);
* this task being preempted, its old CPU being taken offline, resuming
* on some other CPU, then determining that its old CPU is now offline.
* It is OK to use RCU on an offline processor during initial boot, hence
- * the check for rcu_scheduler_fully_active.
+ * the check for rcu_scheduler_fully_active. Note also that it is OK
+ * for a CPU coming online to use RCU for one jiffy prior to marking itself
+ * online in the cpu_online_mask. Similarly, it is OK for a CPU going
+ * offline to continue to use RCU for one jiffy after marking itself
+ * offline in the cpu_online_mask. This leniency is necessary given the
+ * non-atomic nature of the online and offline processing, for example,
+ * the fact that a CPU enters the scheduler after completing the CPU_DYING
+ * notifiers.
+ *
+ * This is also why RCU internally marks CPUs online during the
+ * CPU_UP_PREPARE phase and offline during the CPU_DEAD phase.
*
* Disable checking if in an NMI handler because we cannot safely report
* errors from NMI handlers anyway.
*/
bool rcu_lockdep_current_cpu_online(void)
{
+ struct rcu_data *rdp;
+ struct rcu_node *rnp;
bool ret;
if (in_nmi())
return 1;
preempt_disable();
- ret = cpu_online(smp_processor_id()) ||
+ rdp = &__get_cpu_var(rcu_sched_data);
+ rnp = rdp->mynode;
+ ret = (rdp->grpmask & rnp->qsmaskinit) ||
!rcu_scheduler_fully_active;
preempt_enable();
return ret;
@@ -1308,14 +1315,12 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
*/
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
- unsigned long flags;
int i;
unsigned long mask;
- int need_report;
int receive_cpu = cpumask_any(cpu_online_mask);
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
- struct rcu_node *rnp = rdp->mynode; /* For dying CPU. */
+ RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */
/* First, adjust the counts. */
if (rdp->nxtlist != NULL) {
@@ -1381,32 +1386,6 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
"cpuofl");
rcu_report_qs_rdp(smp_processor_id(), rsp, rdp, rsp->gpnum);
/* Note that rcu_report_qs_rdp() might call trace_rcu_grace_period(). */
-
- /*
- * Remove the dying CPU from the bitmasks in the rcu_node
- * hierarchy. Because we are in stop_machine() context, we
- * automatically exclude ->onofflock critical sections.
- */
- do {
- raw_spin_lock_irqsave(&rnp->lock, flags);
- rnp->qsmaskinit &= ~mask;
- if (rnp->qsmaskinit != 0) {
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- break;
- }
- if (rnp == rdp->mynode) {
- need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
- if (need_report & RCU_OFL_TASKS_NORM_GP)
- rcu_report_unblock_qs_rnp(rnp, flags);
- else
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- if (need_report & RCU_OFL_TASKS_EXP_GP)
- rcu_report_exp_rnp(rsp, rnp, true);
- } else
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
- mask = rnp->grpmask;
- rnp = rnp->parent;
- } while (rnp != NULL);
}
/*
@@ -1417,11 +1396,53 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
*/
static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
{
+ unsigned long flags;
+ unsigned long mask;
+ int need_report = 0;
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
- struct rcu_node *rnp = rdp->mynode;
+ struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rnp. */
+ /* Adjust any no-longer-needed kthreads. */
rcu_stop_cpu_kthread(cpu);
rcu_node_kthread_setaffinity(rnp, -1);
+
+ /* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
+
+ /* Exclude any attempts to start a new grace period. */
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+
+ /* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
+ mask = rdp->grpmask; /* rnp->grplo is constant. */
+ do {
+ raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+ rnp->qsmaskinit &= ~mask;
+ if (rnp->qsmaskinit != 0) {
+ if (rnp != rdp->mynode)
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ break;
+ }
+ if (rnp == rdp->mynode)
+ need_report = rcu_preempt_offline_tasks(rsp, rnp, rdp);
+ else
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ mask = rnp->grpmask;
+ rnp = rnp->parent;
+ } while (rnp != NULL);
+
+ /*
+ * We still hold the leaf rcu_node structure lock here, and
+ * irqs are still disabled. The reason for this subterfuge is
+ * because invoking rcu_report_unblock_qs_rnp() with ->onofflock
+ * held leads to deadlock.
+ */
+ raw_spin_unlock(&rsp->onofflock); /* irqs remain disabled. */
+ rnp = rdp->mynode;
+ if (need_report & RCU_OFL_TASKS_NORM_GP)
+ rcu_report_unblock_qs_rnp(rnp, flags);
+ else
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ if (need_report & RCU_OFL_TASKS_EXP_GP)
+ rcu_report_exp_rnp(rsp, rnp, true);
}
#else /* #ifdef CONFIG_HOTPLUG_CPU */