Messages in this thread Patch in this message | | | From | riel@redhat ... | Subject | [PATCH 2/2] sched,numa: cap pte scanning overhead to 3% of run time | Date | Thu, 5 Nov 2015 15:56:23 -0500 |
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From: Rik van Riel <riel@redhat.com>
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 <riel@redhat.com> Reported-and-tested-by: Jan Stancek <jstancek@redhat.com> --- 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
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