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 <[email protected]>
Reviewed-by: Wanpeng Li <[email protected]>
> ---
> 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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