[PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paul.mckenney@linaro.org>

Some uses of RCU benefit from shorter grace periods, while others benefit
more from the greater efficiency provided by longer grace periods.
Therefore, this commit allows the durations to be controlled from sysfs.
There are two sysfs parameters, one named "jiffies_till_first_fqs" that
specifies the delay in jiffies from the end of grace-period initialization
until the first attempt to force quiescent states, and the other named
"jiffies_till_next_fqs" that specifies the delay (again in jiffies)
between subsequent attempts to force quiescent states.  They both default
to three jiffies, which is compatible with the old hard-coded behavior.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/kernel-parameters.txt |   11 +++++++++++
 kernel/rcutree.c                    |   25 ++++++++++++++++++++++---
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index ad7e2e5..55ada04 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2385,6 +2385,17 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	rcutree.rcu_cpu_stall_timeout= [KNL,BOOT]
 			Set timeout for RCU CPU stall warning messages.
 
+	rcutree.jiffies_till_first_fqs= [KNL,BOOT]
+			Set delay from grace-period initialization to
+			first attempt to force quiescent states.
+			Units are jiffies, minimum value is zero,
+			and maximum value is HZ.
+
+	rcutree.jiffies_till_next_fqs= [KNL,BOOT]
+			Set delay between subsequent attempts to force
+			quiescent states.  Units are jiffies, minimum
+			value is one, and maximum value is HZ.
+
 	rcutorture.fqs_duration= [KNL,BOOT]
 			Set duration of force_quiescent_state bursts.
 
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ed1be62..1d33240 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -226,6 +226,12 @@ int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
 module_param(rcu_cpu_stall_suppress, int, 0644);
 module_param(rcu_cpu_stall_timeout, int, 0644);
 
+static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS;
+static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
+
+module_param(jiffies_till_first_fqs, ulong, 0644);
+module_param(jiffies_till_next_fqs, ulong, 0644);
+
 static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
 static void force_quiescent_state(struct rcu_state *rsp);
 static int rcu_pending(int cpu);
@@ -1193,6 +1199,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 static int rcu_gp_kthread(void *arg)
 {
 	int fqs_state;
+	unsigned long j;
 	int ret;
 	struct rcu_state *rsp = arg;
 	struct rcu_node *rnp = rcu_get_root(rsp);
@@ -1213,14 +1220,18 @@ static int rcu_gp_kthread(void *arg)
 
 		/* Handle quiescent-state forcing. */
 		fqs_state = RCU_SAVE_DYNTICK;
+		j = jiffies_till_first_fqs;
+		if (j > HZ) {
+			j = HZ;
+			jiffies_till_first_fqs = HZ;
+		}
 		for (;;) {
-			rsp->jiffies_force_qs = jiffies +
-						RCU_JIFFIES_TILL_FORCE_QS;
+			rsp->jiffies_force_qs = jiffies + j;
 			ret = wait_event_interruptible_timeout(rsp->gp_wq,
 					(rsp->gp_flags & RCU_GP_FLAG_FQS) ||
 					(!ACCESS_ONCE(rnp->qsmask) &&
 					 !rcu_preempt_blocked_readers_cgp(rnp)),
-					RCU_JIFFIES_TILL_FORCE_QS);
+					j);
 			/* If grace period done, leave loop. */
 			if (!ACCESS_ONCE(rnp->qsmask) &&
 			    !rcu_preempt_blocked_readers_cgp(rnp))
@@ -1234,6 +1245,14 @@ static int rcu_gp_kthread(void *arg)
 				cond_resched();
 				flush_signals(current);
 			}
+			j = jiffies_till_next_fqs;
+			if (j > HZ) {
+				j = HZ;
+				jiffies_till_next_fqs = HZ;
+			} else if (j < 1) {
+				j = 1;
+				jiffies_till_next_fqs = 1;
+			}
 		}
 
 		/* Handle grace-period end. */
-- 
1.7.8