(2014/10/22 16:09), Wanpeng Li wrote:
> 10/22/14, 3:04 PM, Yasuaki Ishimatsu:
>> 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.
>> This patch always increments denominator for avoiding divide error.
>>
>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasu...@jp.fujitsu.com>
>
> Reviewed-by: Wanpeng Li <wanpeng...@linux.intel.com>
Thank you for your review.
Thanks,
Yasuaki Ishimatsu
>
>> ---
>> v2:
>> - Simply increment a denominator
>>
>> kernel/sched/fair.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 0b069bf..f3b492d 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1520,7 +1520,7 @@ 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, (private +
>> shared + 1));
>> diff = (diff * ratio) / NUMA_PERIOD_SLOTS;
>> }
>>
>
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