From: Rik van Riel <[email protected]> There is a fundamental mismatch between the runtime based NUMA scanning at the task level, and the wall clock time NUMA scanning at the mm level. On a severely overloaded system, with very large processes, this mismatch can cause the system to spend all of its time in change_prot_numa().
This can happen if the task spends at least two ticks in change_prot_numa(), and only gets two ticks of CPU time in the real time between two scan intervals of the mm. This patch ensures that a task never spends more than 3% of run time scanning PTEs. It does that by ensuring that in-between task_numa_work runs, the task spends at least 32x as much time on other things than it did on task_numa_work. This is done stochastically: if a timer tick happens, or the task gets rescheduled during task_numa_work, we delay a future run of task_numa_work until the task has spent at least 32x the amount of CPU time doing something else, as it spent inside task_numa_work. The longer task_numa_work takes, the more likely it is this happens. If task_numa_work takes very little time, chances are low that that code will do anything, but we will not care. Signed-off-by: Rik van Riel <[email protected]> Reported-and-tested-by: Jan Stancek <[email protected]> --- kernel/sched/fair.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index f04fda8f669c..b0924377ab0d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2155,6 +2155,7 @@ void task_numa_work(struct callback_head *work) unsigned long migrate, next_scan, now = jiffies; struct task_struct *p = current; struct mm_struct *mm = p->mm; + u64 runtime = p->se.sum_exec_runtime; struct vm_area_struct *vma; unsigned long start, end; unsigned long nr_pte_updates = 0; @@ -2277,6 +2278,17 @@ void task_numa_work(struct callback_head *work) else reset_ptenuma_scan(p); up_read(&mm->mmap_sem); + + /* + * Make sure tasks use at least 32x as much time to run other code + * than they used here, to limit NUMA PTE scanning overhead to 3% max. + * Usually update_task_scan_period slows down scanning enough; on an + * overloaded system we need to limit overhead on a per task basis. + */ + if (unlikely(p->se.sum_exec_runtime != runtime)) { + u64 diff = p->se.sum_exec_runtime - runtime; + p->node_stamp += 32 * diff; + } } /* -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
