Re: [PATCH] sched,numa cap pte scanning overhead to 3% of run time

[Rik van Riel <riel@surriel.com>]

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.