[PATCH tip/core/rcu 01/15] sched,rcu: Make cond_resched() provide RCU quiescent state

[Paul E. McKenney]

There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU quiescent
states to cond_resched().  This commit also simplifies the code that
used to interact with cond_resched_rcu_qs(), and that now interacts with
cond_resched(), to reduce its overhead.  This reduction is necessary to
allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
everywhere that cond_resched() is invoked.

Part of that reduction in overhead converts the jiffies_till_sched_qs
kernel parameter to read-only at runtime, thus eliminating the need for
bounds checking.

Reported-by: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 include/linux/sched.h |  3 ++-
 kernel/rcu/tree.c     | 23 ++++-------------------
 kernel/sched/core.c   |  1 +
 3 files changed, 7 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2ba9ec93423f..12f326aa5871 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1508,10 +1508,11 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
  * cond_resched_lock() will drop the spinlock before scheduling,
  * cond_resched_softirq() will enable bhs before scheduling.
  */
+void rcu_all_qs(void);
 #ifndef CONFIG_PREEMPT
 extern int _cond_resched(void);
 #else
-static inline int _cond_resched(void) { return 0; }
+static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
 #endif
 
 #define cond_resched() ({			\
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51d4c3acf32d..ab4c2cea208d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -537,8 +537,8 @@ module_param(rcu_kick_kthreads, bool, 0644);
  * How long the grace period must be before we start recruiting
  * quiescent-state help from rcu_note_context_switch().
  */
-static ulong jiffies_till_sched_qs = HZ / 20;
-module_param(jiffies_till_sched_qs, ulong, 0644);
+static ulong jiffies_till_sched_qs = HZ / 16;
+module_param(jiffies_till_sched_qs, ulong, 0444);
 
 static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 				  struct rcu_data *rdp);
@@ -1230,7 +1230,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	unsigned long jtsq;
 	bool *rnhqp;
 	bool *ruqp;
-	unsigned long rjtsc;
 	struct rcu_node *rnp;
 
 	/*
@@ -1247,23 +1246,13 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 		return 1;
 	}
 
-	/* Compute and saturate jiffies_till_sched_qs. */
-	jtsq = jiffies_till_sched_qs;
-	rjtsc = rcu_jiffies_till_stall_check();
-	if (jtsq > rjtsc / 2) {
-		WRITE_ONCE(jiffies_till_sched_qs, rjtsc);
-		jtsq = rjtsc / 2;
-	} else if (jtsq < 1) {
-		WRITE_ONCE(jiffies_till_sched_qs, 1);
-		jtsq = 1;
-	}
-
 	/*
 	 * Has this CPU encountered a cond_resched_rcu_qs() since the
 	 * beginning of the grace period?  For this to be the case,
 	 * the CPU has to have noticed the current grace period.  This
 	 * might not be the case for nohz_full CPUs looping in the kernel.
 	 */
+	jtsq = jiffies_till_sched_qs;
 	rnp = rdp->mynode;
 	ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
 	if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
@@ -1271,7 +1260,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	    READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
 		return 1;
-	} else {
+	} else if (time_after(jiffies, rdp->rsp->gp_start + jtsq)) {
 		/* Load rcu_qs_ctr before store to rcu_urgent_qs. */
 		smp_store_release(ruqp, true);
 	}
@@ -1299,10 +1288,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	 * updates are only once every few jiffies, the probability of
 	 * lossage (and thus of slight grace-period extension) is
 	 * quite low.
-	 *
-	 * Note that if the jiffies_till_sched_qs boot/sysfs parameter
-	 * is set too high, we override with half of the RCU CPU stall
-	 * warning delay.
 	 */
 	rnhqp = &per_cpu(rcu_dynticks.rcu_need_heavy_qs, rdp->cpu);
 	if (!READ_ONCE(*rnhqp) &&
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 17c667b427b4..9433633012ba 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4808,6 +4808,7 @@ int __sched _cond_resched(void)
 		preempt_schedule_common();
 		return 1;
 	}
+	rcu_all_qs();
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);
-- 
2.5.2