sched/fair: Care divide error in update_task_scan_period()
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: NYasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com> Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/54475703.8000505@jp.fujitsu.comSigned-off-by: NIngo Molnar <mingo@kernel.org>
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