Re: Fw: [lkp-developer] [sched,rcu] cf7a2dca60: [No primary change] +186% will-it-scale.time.involuntary_context_switches

[Michal Hocko]

On Wed 14-12-16 03:06:09, Paul E. McKenney wrote:
> On Wed, Dec 14, 2016 at 10:54:25AM +0100, Michal Hocko wrote:
> > On Tue 13-12-16 07:14:08, Paul E. McKenney wrote:
> > > Just FYI for the moment...
> > > 
> > > So even with the slowed-down checking, making cond_resched() do what
> > > cond_resched_rcu_qs() does results in a smallish but quite measurable
> > > degradation according to 0day.
> > 
> > So if I understand those results properly, the reason seems to be the
> > increased involuntary context switches, right? Or am I misreading the
> > data?
> > I am looking at your "sched,rcu: Make cond_resched() provide RCU
> > quiescent state" in linux-next and I am wondering whether rcu_all_qs has
> > to be called unconditionally and not only when should_resched failed few
> > times? I guess you have discussed that with Peter already but do not
> > remember the outcome.
> 
> My first thought is to wait for the grace period to age further before
> checking, the idea being to avoid increasing cond_resched() overhead
> any further.  But if that doesn't work, then yes, I may have to look at
> adding more checks to cond_resched().

This might be really naive but would something like the following work?
The overhead should be pretty much negligible, I guess. Ideally the pcp
variable could be set somewhere from check_cpu_stall() but I couldn't
wrap my head around that code to see how exactly.
--- 
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index ac81e4063b40..1c005c5304a3 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -243,6 +243,10 @@ static inline void rcu_all_qs(void)
 	barrier(); /* Avoid RCU read-side critical sections leaking across. */
 }
 
+static inline void cond_resched_rcu_check(void)
+{
+}
+
 /* RCUtree hotplug events */
 #define rcutree_prepare_cpu      NULL
 #define rcutree_online_cpu       NULL
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 63a4e4cf40a5..176f6e386379 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -110,6 +110,18 @@ extern int rcu_scheduler_active __read_mostly;
 bool rcu_is_watching(void);
 
 void rcu_all_qs(void);
+#ifndef CONFIG_PREEMPT
+DECLARE_PER_CPU(int, rcu_needs_qs);
+
+static inline void cond_resched_rcu_check(void)
+{
+	/* Make sure we do not miss rcu_all_qs at least every now and then */
+	if (this_cpu_inc_return(rcu_needs_qs) > 10) {
+		this_cpu_write(rcu_needs_qs, 0);
+		rcu_all_qs();
+	}
+}
+#endif
 
 /* RCUtree hotplug events */
 int rcutree_prepare_cpu(unsigned int cpu);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 69a5611a7e7c..783c74ae9930 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -268,6 +268,9 @@ void rcu_bh_qs(void)
 }
 
 static DEFINE_PER_CPU(int, rcu_sched_qs_mask);
+#ifndef CONFIG_PREEMPT
+DEFINE_PER_CPU(int, rcu_needs_qs);
+#endif
 
 static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 154fd689fe02..a58844be2ef1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4905,6 +4905,8 @@ int __sched _cond_resched(void)
 	if (should_resched(0)) {
 		preempt_schedule_common();
 		return 1;
+	} else {
+		cond_resched_rcu_check();
 	}
 	return 0;
 }
-- 
Michal Hocko
SUSE Labs