time: RCU permitted to stop idle entry via softirq

The can_stop_idle_tick() function complains if a softirq vector is
raised too late in the idle-entry process, presumably in order to
prevent dangling softirq invocations from being delayed across the
full idle period, which might be indefinitely long -- and if softirq
was asserted any later than the call to this function, such a delay
might well happen.

However, RCU needs to be able to use softirq to stop idle entry in
order to be able to drain RCU callbacks from the current CPU, which in
turn enables faster entry into dyntick-idle mode, which in turn reduces
power consumption.  Because RCU takes this action at a well-defined
point in the idle-entry path, it is safe for RCU to take this approach.

This commit therefore silences the error message that is sometimes
produced when the going-idle CPU suddenly finds that it has an RCU_SOFTIRQ
to process.  The error message will continue to be issued for other
softirq vectors.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

2 files changed, 4 insertions, 1 deletions

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index c5f856a040b9..5e4e6170f43a 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -430,6 +430,8 @@ enum
         NR_SOFTIRQS
 };
 
+#define SOFTIRQ_STOP_IDLE_MASK (~(1 << RCU_SOFTIRQ))
+
 /* map softirq index to softirq name. update 'softirq_to_name' in
  * kernel/softirq.c when adding a new softirq.
  */
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 024540f97f74..4b1785a7bb83 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -436,7 +436,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
         if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
                 static int ratelimit;
 
-                if (ratelimit < 10) {
+                if (ratelimit < 10 &&
+                    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
                         printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
                                (unsigned int) local_softirq_pending());
                         ratelimit++;