Messages in this thread | | | Subject | Re: [PATCH] sched,numa cap pte scanning overhead to 3% of run time | From | Rik van Riel <> | Date | Thu, 5 Nov 2015 10:56:29 -0500 |
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On 11/05/2015 10:34 AM, Peter Zijlstra wrote: > On Wed, Nov 04, 2015 at 01:25:15PM -0500, Rik van Riel wrote: >> +++ 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 +2278,20 @@ void task_numa_work(struct callback_head *work) >> else >> reset_ptenuma_scan(p); >> up_read(&mm->mmap_sem); >> + >> + /* >> + * 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(). >> + * Limit NUMA PTE scanning to 3% of the task's run time, if >> + * we spent so much time scanning we got rescheduled. >> + */ >> + if (unlikely(p->se.sum_exec_runtime != runtime)) { >> + u64 diff = p->se.sum_exec_runtime - runtime; >> + p->node_stamp += 32 * diff; >> + } > > I don't actually see how this does what it says it does
If we got rescheduled during the assigning of runtime above, and this point, the scheduler should have updated the p->se.sum_exec_runtime statistic, given that update_curr is called from both dequeue_entity and enqueue_entity in fair.c
Advancing the node_stamp by 32x the amount of time the task consumed between entering task_numa_work and this point should ensure task_numa_work does not get queued again until we have used 32x as much time doing something else.
That should limit the CPU time used by task_numa_work.
What am I missing?
>> @@ -2302,7 +2317,7 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr) >> now = curr->se.sum_exec_runtime; >> period = (u64)curr->numa_scan_period * NSEC_PER_MSEC; >> >> - if (now - curr->node_stamp > period) { >> + if (now > curr->node_stamp + period) { >> if (!curr->node_stamp) >> curr->numa_scan_period = task_scan_min(curr); >> curr->node_stamp += period; > > And this really should be an independent patch. Although the fix I had > in mind looked like: > > if ((s64)(now - curr->node_stamp) > period) > > But I suppose this works too.
I can resend this as a separate patch if you prefer.
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