rcu: refactor RCU's context-switch handling
The addition of preemptible RCU to treercu resulted in a bit of
confusion and inefficiency surrounding the handling of context switches
for RCU-sched and for RCU-preempt. For RCU-sched, a context switch
is a quiescent state, pure and simple, just like it always has been.
For RCU-preempt, a context switch is in no way a quiescent state, but
special handling is required when a task blocks in an RCU read-side
critical section.
However, the callout from the scheduler and the outer loop in ksoftirqd
still calls something named rcu_sched_qs(), whose name is no longer
accurate. Furthermore, when rcu_check_callbacks() notes an RCU-sched
quiescent state, it ends up unnecessarily (though harmlessly, aside
from the performance hit) enqueuing the current task if it happens to
be running in an RCU-preempt read-side critical section. This not only
increases the maximum latency of scheduler_tick(), it also needlessly
increases the overhead of the next outermost rcu_read_unlock() invocation.
This patch addresses this situation by separating the notion of RCU's
context-switch handling from that of RCU-sched's quiescent states.
The context-switch handling is covered by rcu_note_context_switch() in
general and by rcu_preempt_note_context_switch() for preemptible RCU.
This permits rcu_sched_qs() to handle quiescent states and only quiescent
states. It also reduces the maximum latency of scheduler_tick(), though
probably by much less than a microsecond. Finally, it means that tasks
within preemptible-RCU read-side critical sections avoid incurring the
overhead of queuing unless there really is a context switch.
Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index bbeb55b..ff22b97 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -29,6 +29,10 @@
void rcu_sched_qs(int cpu);
void rcu_bh_qs(int cpu);
+static inline void rcu_note_context_switch(int cpu)
+{
+ rcu_sched_qs(cpu);
+}
#define __rcu_read_lock() preempt_disable()
#define __rcu_read_unlock() preempt_enable()
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 7484fe6..b9f7460 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -34,6 +34,7 @@
extern void rcu_sched_qs(int cpu);
extern void rcu_bh_qs(int cpu);
+extern void rcu_note_context_switch(int cpu);
extern int rcu_needs_cpu(int cpu);
extern int rcu_expedited_torture_stats(char *page);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 86bb949..e336313 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -97,25 +97,32 @@
*/
void rcu_sched_qs(int cpu)
{
- struct rcu_data *rdp;
+ struct rcu_data *rdp = &per_cpu(rcu_sched_data, cpu);
- rdp = &per_cpu(rcu_sched_data, cpu);
rdp->passed_quiesc_completed = rdp->gpnum - 1;
barrier();
rdp->passed_quiesc = 1;
- rcu_preempt_note_context_switch(cpu);
}
void rcu_bh_qs(int cpu)
{
- struct rcu_data *rdp;
+ struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
- rdp = &per_cpu(rcu_bh_data, cpu);
rdp->passed_quiesc_completed = rdp->gpnum - 1;
barrier();
rdp->passed_quiesc = 1;
}
+/*
+ * Note a context switch. This is a quiescent state for RCU-sched,
+ * and requires special handling for preemptible RCU.
+ */
+void rcu_note_context_switch(int cpu)
+{
+ rcu_sched_qs(cpu);
+ rcu_preempt_note_context_switch(cpu);
+}
+
#ifdef CONFIG_NO_HZ
DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
.dynticks_nesting = 1,
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 687c4e9..f9bc83a 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -75,13 +75,19 @@
* that this just means that the task currently running on the CPU is
* not in a quiescent state. There might be any number of tasks blocked
* while in an RCU read-side critical section.
+ *
+ * Unlike the other rcu_*_qs() functions, callers to this function
+ * must disable irqs in order to protect the assignment to
+ * ->rcu_read_unlock_special.
*/
static void rcu_preempt_qs(int cpu)
{
struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
+
rdp->passed_quiesc_completed = rdp->gpnum - 1;
barrier();
rdp->passed_quiesc = 1;
+ current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
}
/*
@@ -144,9 +150,8 @@
* grace period, then the fact that the task has been enqueued
* means that we continue to block the current grace period.
*/
- rcu_preempt_qs(cpu);
local_irq_save(flags);
- t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
+ rcu_preempt_qs(cpu);
local_irq_restore(flags);
}
@@ -236,7 +241,6 @@
*/
special = t->rcu_read_unlock_special;
if (special & RCU_READ_UNLOCK_NEED_QS) {
- t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
rcu_preempt_qs(smp_processor_id());
}
@@ -473,7 +477,6 @@
struct task_struct *t = current;
if (t->rcu_read_lock_nesting == 0) {
- t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
rcu_preempt_qs(cpu);
return;
}
diff --git a/kernel/sched.c b/kernel/sched.c
index 3c2a54f..d8a213c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3706,7 +3706,7 @@
preempt_disable();
cpu = smp_processor_id();
rq = cpu_rq(cpu);
- rcu_sched_qs(cpu);
+ rcu_note_context_switch(cpu);
prev = rq->curr;
switch_count = &prev->nivcsw;
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 7c1a67e..0db913a 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -716,7 +716,7 @@
preempt_enable_no_resched();
cond_resched();
preempt_disable();
- rcu_sched_qs((long)__bind_cpu);
+ rcu_note_context_switch((long)__bind_cpu);
}
preempt_enable();
set_current_state(TASK_INTERRUPTIBLE);
