Messages in this thread | | | Date | Thu, 5 Nov 2015 17:37:12 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched,numa cap pte scanning overhead to 3% of run time |
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On Thu, Nov 05, 2015 at 10:56:29AM -0500, Rik van Riel wrote: > 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
Or just had a tick. Even if the whole thing took a fraction of a ms but we got unlucky and got hit by a tick the sum_exec_runtime would get updated and not match here.
> 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.
> What am I missing?
The above, issue and the fact that I'm really tired and didn't do 1:32 ~ 3%.
So the tick scenario can cause a 32*TICK_NSEC delay even though we spend much less than TICK_NSEC time scanning, dropping th effective rate much below the 3%.
Not sure it makes sense to do more accurate accounting, but I suppose we should mention it somewhere.
> >> @@ -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;
> I can resend this as a separate patch if you prefer.
Yes, its an unrelated fix.
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