Could you review this patch?
(2014/10/16 18:48), Yasuaki Ishimatsu wrote:
> 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.yasu...@jp.fujitsu.com>
> CC: Wanpeng Li <kernel...@gmail.com>
> CC: Rik van Riel <r...@redhat.com>
> CC: Peter Zijlstra <pet...@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;
> }
>
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