rcu: Add softirq-stall indications to stall-warning messages

If RCU's softirq handler is prevented from executing, an RCU CPU stall
warning can result.  Ways to prevent RCU's softirq handler from executing
include: (1) CPU spinning with interrupts disabled, (2) infinite loop
in some softirq handler, and (3) in -rt kernels, an infinite loop in a
set of real-time threads running at priorities higher than that of RCU's
softirq handler.

Because this situation can be difficult to track down, this commit causes
the count of RCU softirq handler invocations to be printed with RCU
CPU stall warnings.  This information does require some interpretation,
as now documented in Documentation/RCU/stallwarn.txt.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c1cc7e17ff9d..7fcd3bbf67da 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2070,10 +2070,11 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
                 ticks_value = rsp->gpnum - rdp->gpnum;
         }
         print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
-        printk(KERN_ERR "\t%d: (%lu %s) idle=%03x/%llx/%d %s\n",
+        printk(KERN_ERR "\t%d: (%lu %s) idle=%03x/%llx/%d softirq=%u/%u %s\n",
                cpu, ticks_value, ticks_title,
                atomic_read(&rdtp->dynticks) & 0xfff,
                rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,
+               rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
                fast_no_hz);
 }
 
@@ -2087,6 +2088,7 @@ static void print_cpu_stall_info_end(void)
 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
 {
         rdp->ticks_this_gp = 0;
+        rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
 }
 
 /* Increment ->ticks_this_gp for all flavors of RCU. */