rcu: Set RCU CPU stall times via sysfs

The default CONFIG_RCU_CPU_STALL_TIMEOUT value of 60 seconds has served
Linux users well for production use for quite some time.  However, for
debugging, there will be more than three minutes between subsequent
stall-warning messages.  This can be an annoyingly long wait if you
are trying to work out where the offending infinite loop is hiding.

Therefore, this commit provides a rcu_cpu_stall_timeout sysfs
parameter that may be adjusted at boot time and at runtime to speed
up debugging.

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

2 files changed, 26 insertions, 11 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 61adb351d2c7..cfdab9898e33 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -208,8 +208,11 @@ module_param(blimit, int, 0);
 module_param(qhimark, int, 0);
 module_param(qlowmark, int, 0);
 
-int rcu_cpu_stall_suppress __read_mostly;
+int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */
+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 void force_quiescent_state(struct rcu_state *rsp, int relaxed);
 static int rcu_pending(int cpu);
@@ -645,10 +648,28 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
         return rcu_implicit_offline_qs(rdp);
 }
 
+static int jiffies_till_stall_check(void)
+{
+        int till_stall_check = ACCESS_ONCE(rcu_cpu_stall_timeout);
+
+        /*
+         * Limit check must be consistent with the Kconfig limits
+         * for CONFIG_RCU_CPU_STALL_TIMEOUT.
+         */
+        if (till_stall_check < 3) {
+                ACCESS_ONCE(rcu_cpu_stall_timeout) = 3;
+                till_stall_check = 3;
+        } else if (till_stall_check > 300) {
+                ACCESS_ONCE(rcu_cpu_stall_timeout) = 300;
+                till_stall_check = 300;
+        }
+        return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
+}
+
 static void record_gp_stall_check_time(struct rcu_state *rsp)
 {
         rsp->gp_start = jiffies;
-        rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
+        rsp->jiffies_stall = jiffies + jiffies_till_stall_check();
 }
 
 static void print_other_cpu_stall(struct rcu_state *rsp)
@@ -667,7 +688,7 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 return;
         }
-        rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+        rsp->jiffies_stall = jiffies + 3 * jiffies_till_stall_check() + 3;
 
         /*
          * Now rat on any tasks that got kicked up to the root rcu_node
@@ -726,8 +747,8 @@ static void print_cpu_stall(struct rcu_state *rsp)
 
         raw_spin_lock_irqsave(&rnp->lock, flags);
         if (ULONG_CMP_GE(jiffies, rsp->jiffies_stall))
-                rsp->jiffies_stall =
-                        jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+                rsp->jiffies_stall = jiffies +
+                                     3 * jiffies_till_stall_check() + 3;
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
 
         set_need_resched();  /* kick ourselves to get things going. */
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 58c9fc3bc820..0328a537846a 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -314,12 +314,6 @@ struct rcu_data {
 #else
 #define RCU_STALL_DELAY_DELTA          0
 #endif
-
-#define RCU_SECONDS_TILL_STALL_CHECK   (CONFIG_RCU_CPU_STALL_TIMEOUT * HZ + \
-                                        RCU_STALL_DELAY_DELTA)
-                                                /* for rsp->jiffies_stall */
-#define RCU_SECONDS_TILL_STALL_RECHECK (3 * RCU_SECONDS_TILL_STALL_CHECK + 30)
-                                                /* for rsp->jiffies_stall */
 #define RCU_STALL_RAT_DELAY             2       /* Allow other CPUs time */
                                                 /*  to take at least one */
                                                 /*  scheduling clock irq */