Commit ac1bea85 authored by Paul E. McKenney's avatar Paul E. McKenney

sched,rcu: Make cond_resched() report RCU quiescent states

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent 0e980234
......@@ -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 */
......@@ -286,6 +287,41 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
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.
......
......@@ -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();
}
......@@ -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();
......
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