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[PATCH tip/core/rcu 04/20] rcu: Only dump stalled-tasks stacks if there was a real stall

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From: Byungchul Park <byungchul.park@lge.com>

The print_other_cpu_stall() function currently unconditionally invokes
rcu_print_detail_task_stall().  This is OK because if there was a stall
sufficient to cause print_other_cpu_stall() to be invoked, that stall
is very likely to persist through the entire print_other_cpu_stall()
execution.  However, if the stall did not persist, the variable ndetected
will be zero, and that variable is already tested in an "if" statement.
Therefore, this commit moves the call to rcu_print_detail_task_stall()
under that pre-existing "if" to improve readability, with a very rare
reduction in overhead.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
[ paulmck: Reworked commit log. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cb4e2056ccf3..6232d2f9a84e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1379,6 +1379,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	       (long)rsp->gpnum, (long)rsp->completed, totqlen);
 	if (ndetected) {
 		rcu_dump_cpu_stacks(rsp);
+
+		/* Complain about tasks blocking the grace period. */
+		rcu_print_detail_task_stall(rsp);
 	} else {
 		if (READ_ONCE(rsp->gpnum) != gpnum ||
 		    READ_ONCE(rsp->completed) == gpnum) {
@@ -1395,9 +1398,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 		}
 	}
 
-	/* Complain about tasks blocking the grace period. */
-	rcu_print_detail_task_stall(rsp);
-
 	rcu_check_gp_kthread_starvation(rsp);
 
 	panic_on_rcu_stall();
-- 
2.5.2

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[PATCH tip/core/rcu 0/20] Miscellaneous fixes for 4.11 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 19/20] rcu: Eliminate unused expedited_normal counter "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 12/20] rcu: Re-enable TASKS_RCU for User Mode Linux "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 20/20] rcu: Add lockdep checks to synchronous expedited primitives "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 16/20] rcu: Enable RCU tracepoints by default to aid in debugging "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 03/20] Fix: Disable sys_membarrier when nohz_full is enabled "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 17/20] rcu: Fix comment in rcu_organize_nocb_kthreads() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 10/20] rcu: Add functions to test for trivial grace periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
    Re: [PATCH tip/core/rcu 10/20] rcu: Add functions to test for  trivial grace periods Josh Triplett <josh@joshtriplett.org> - 2017-01-16 09:10 +0100
  [PATCH tip/core/rcu 04/20] rcu: Only dump stalled-tasks stacks if there was a real stall "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 18/20] llist: Clarify comments about when locking is needed "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 15/20] rcu: Make rcu_cpu_starting() use its "cpu" argument "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 05/20] rcu: Remove unneeded rcu_process_callbacks() declarations "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  [PATCH tip/core/rcu 01/20] rcu: update: Make RCU_EXPEDITE_BOOT be the default "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
  Re: [PATCH tip/core/rcu 0/20] Miscellaneous fixes for 4.11 Josh Triplett <josh@joshtriplett.org> - 2017-01-16 09:20 +0100

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