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Date: Mon, 28 Apr 2014 17:25:17 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
oleg@...hat.com, sbw@....edu,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 29/45] sched,rcu: Make cond_resched() report RCU quiescent states
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Given a CPU running a loop containing cond_resched(), with no
other tasks runnable on that CPU, RCU will eventually report RCU
CPU stall warnings due to lack of quiescent states. Fortunately,
every call to cond_resched() is a perfectly good quiescent state.
Unfortunately, invoking rcu_note_context_switch() is a bit heavyweight
for cond_resched(), especially given the need to disable preemption,
and, for RCU-preempt, interrupts as well.
This commit therefore maintains a per-CPU counter that causes
cond_resched(), cond_resched_lock(), and cond_resched_softirq() to call
rcu_note_context_switch(), but only about once per 256 invocations.
This ratio was chosen in keeping with the relative time constants of
RCU grace periods.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
include/linux/rcupdate.h | 36 ++++++++++++++++++++++++++++++++++++
kernel/rcu/update.c | 18 ++++++++++++++++++
kernel/sched/core.c | 7 ++++++-
3 files changed, 60 insertions(+), 1 deletion(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index a6c3898e141e..baead1bd0556 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -44,6 +44,7 @@
#include <linux/debugobjects.h>
#include <linux/bug.h>
#include <linux/compiler.h>
+#include <linux/percpu.h>
#include <asm/barrier.h>
extern int rcu_expedited; /* for sysctl */
@@ -287,6 +288,41 @@ bool __rcu_is_watching(void);
#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
/*
+ * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
+ */
+
+#define RCU_COND_RESCHED_LIM 256 /* ms vs. 100s of ms. */
+DECLARE_PER_CPU(int, rcu_cond_resched_count);
+void rcu_resched(void);
+
+/*
+ * Is it time to report RCU quiescent states?
+ *
+ * Note unsynchronized access to rcu_cond_resched_count. Yes, we might
+ * increment some random CPU's count, and possibly also load the result from
+ * yet another CPU's count. We might even clobber some other CPU's attempt
+ * to zero its counter. This is all OK because the goal is not precision,
+ * but rather reasonable amortization of rcu_note_context_switch() overhead
+ * and extremely high probability of avoiding RCU CPU stall warnings.
+ * Note that this function has to be preempted in just the wrong place,
+ * many thousands of times in a row, for anything bad to happen.
+ */
+static inline bool rcu_should_resched(void)
+{
+ return raw_cpu_inc_return(rcu_cond_resched_count) >=
+ RCU_COND_RESCHED_LIM;
+}
+
+/*
+ * Report quiscent states to RCU if it is time to do so.
+ */
+static inline void rcu_cond_resched(void)
+{
+ if (unlikely(rcu_should_resched()))
+ rcu_resched();
+}
+
+/*
* Infrastructure to implement the synchronize_() primitives in
* TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
*/
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 4c0a9b0af469..ed7a0d72562c 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -338,3 +338,21 @@ static int __init check_cpu_stall_init(void)
early_initcall(check_cpu_stall_init);
#endif /* #ifdef CONFIG_RCU_STALL_COMMON */
+
+/*
+ * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
+ */
+
+DEFINE_PER_CPU(int, rcu_cond_resched_count);
+
+/*
+ * Report a set of RCU quiescent states, for use by cond_resched()
+ * and friends. Out of line due to being called infrequently.
+ */
+void rcu_resched(void)
+{
+ preempt_disable();
+ __this_cpu_write(rcu_cond_resched_count, 0);
+ rcu_note_context_switch(smp_processor_id());
+ preempt_enable();
+}
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 268a45ea238c..9f530c9ed911 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4051,6 +4051,7 @@ static void __cond_resched(void)
int __sched _cond_resched(void)
{
+ rcu_cond_resched();
if (should_resched()) {
__cond_resched();
return 1;
@@ -4069,15 +4070,18 @@ EXPORT_SYMBOL(_cond_resched);
*/
int __cond_resched_lock(spinlock_t *lock)
{
+ bool need_rcu_resched = rcu_should_resched();
int resched = should_resched();
int ret = 0;
lockdep_assert_held(lock);
- if (spin_needbreak(lock) || resched) {
+ if (spin_needbreak(lock) || resched || need_rcu_resched) {
spin_unlock(lock);
if (resched)
__cond_resched();
+ else if (unlikely(need_rcu_resched))
+ rcu_resched();
else
cpu_relax();
ret = 1;
@@ -4091,6 +4095,7 @@ int __sched __cond_resched_softirq(void)
{
BUG_ON(!in_softirq());
+ rcu_cond_resched(); /* BH disabled OK, just recording QSes. */
if (should_resched()) {
local_bh_enable();
__cond_resched();
--
1.8.1.5
--
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