[PATCH v2] sched/fair: Care divide error in update_task_scan_period()

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

While offling node by hot removing memory, the following divide error
occurs:

  divide error: 0000 [#1] SMP
  [...]
  Call Trace:
   [...] handle_mm_fault
   [...] ? try_to_wake_up
   [...] ? wake_up_state
   [...] __do_page_fault
   [...] ? do_futex
   [...] ? put_prev_entity
   [...] ? __switch_to
   [...] do_page_fault
   [...] page_fault
  [...]
  RIP  [<ffffffff810a7081>] task_numa_fault
   RSP <ffff88084eb2bcb0>

The issue occurs as follows:
  1. When page fault occurs and page is allocated from node 1,
     task_struct->numa_faults_buffer_memory[] of node 1 is
     incremented and p->numa_faults_locality[] is also incremented
     as follows:

     o numa_faults_buffer_memory[]       o numa_faults_locality[]
              NR_NUMA_HINT_FAULT_TYPES
             |      0     |     1     |
     ----------------------------------  ----------------------
      node 0 |      0     |     0     |   remote |      0     |
      node 1 |      0     |     1     |   locale |      1     |
     ----------------------------------  ----------------------

  2. node 1 is offlined by hot removing memory.

  3. When page fault occurs, fault_types[] is calculated by using
     p->numa_faults_buffer_memory[] of all online nodes in
     task_numa_placement(). But node 1 was offline by step 2. So
     the fault_types[] is calculated by using only
     p->numa_faults_buffer_memory[] of node 0. So both of fault_types[]
     are set to 0.

  4. The values(0) of fault_types[] pass to update_task_scan_period().

  5. numa_faults_locality[1] is set to 1. So the following division is
     calculated.

        static void update_task_scan_period(struct task_struct *p,
                                unsigned long shared, unsigned long private){
        ...
                ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared));
        }

  6. But both of private and shared are set to 0. So divide error
     occurs here.

The divide error is rare case because the trigger is node offline.
By this patch, when both of private and shared are set to 0,
denominator is set to 1 for avoiding divide error.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
CC: Wanpeng Li <kernellwp@gmail.com>
CC: Rik van Riel <riel@redhat.com>
CC: Peter Zijlstra <peterz@infradead.org>
---
 kernel/sched/fair.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bfa3c86..580fc74 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1466,6 +1466,7 @@ static void update_task_scan_period(struct task_struct *p,

 	unsigned long remote = p->numa_faults_locality[0];
 	unsigned long local = p->numa_faults_locality[1];
+	unsigned long total_faults = shared + private;

 	/*
 	 * If there were no record hinting faults then either the task is
@@ -1496,6 +1497,14 @@ static void update_task_scan_period(struct task_struct *p,
 			slot = 1;
 		diff = slot * period_slot;
 	} else {
+		/*
+		 * This is a rare case. total_faults might become 0 after
+		 * offlining node. In this case, total_faults is set to 1
+		 * for avoiding divide error.
+		 */
+		if (unlikely(total_faults == 0))
+			total_faults = 1;
+
 		diff = -(NUMA_PERIOD_THRESHOLD - ratio) * period_slot;

 		/*
@@ -1506,7 +1515,7 @@ static void update_task_scan_period(struct task_struct *p,
 		 * scanning faster if shared accesses dominate as it may
 		 * simply bounce migrations uselessly
 		 */
-		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (private + shared));
+		ratio = DIV_ROUND_UP(private * NUMA_PERIOD_SLOTS, (total_faults));
 		diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
 	}

-- 
1.8.3.1