rcu: Reduce overhead of cond_resched() checks for RCU

Commit ac1bea85781e (Make cond_resched() report RCU quiescent states)
fixed a problem where a CPU looping in the kernel with but one runnable
task would give RCU CPU stall warnings, even if the in-kernel loop
contained cond_resched() calls.  Unfortunately, in so doing, it introduced
performance regressions in Anton Blanchard's will-it-scale "open1" test.
The problem appears to be not so much the increased cond_resched() path
length as an increase in the rate at which grace periods complete, which
increased per-update grace-period overhead.

This commit takes a different approach to fixing this bug, mainly by
moving the RCU-visible quiescent state from cond_resched() to
rcu_note_context_switch(), and by further reducing the check to a
simple non-zero test of a single per-CPU variable.  However, this
approach requires that the force-quiescent-state processing send
resched IPIs to the offending CPUs.  These will be sent only once
the grace period has reached an age specified by the boot/sysfs
parameter rcutree.jiffies_till_sched_qs, or once the grace period
reaches an age halfway to the point at which RCU CPU stall warnings
will be emitted, whichever comes first.

Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Made rcu_momentary_dyntick_idle() as suggested by the
  ktest build robot.  Also fixed smp_mb() comment as noted by
  Oleg Nesterov. ]

Merge with e552592e (Reduce overhead of cond_resched() checks for RCU)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 1 insertions, 6 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3bdf01b494fe..bc1638b33449 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4147,7 +4147,6 @@ static void __cond_resched(void)
 
 int __sched _cond_resched(void)
 {
-        rcu_cond_resched();
         if (should_resched()) {
                 __cond_resched();
                 return 1;
@@ -4166,18 +4165,15 @@ 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 || need_rcu_resched) {
+        if (spin_needbreak(lock) || resched) {
                 spin_unlock(lock);
                 if (resched)
                         __cond_resched();
-                else if (unlikely(need_rcu_resched))
-                        rcu_resched();
                 else
                         cpu_relax();
                 ret = 1;
@@ -4191,7 +4187,6 @@ 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();