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

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Date:	Thu, 5 Nov 2015 16:34:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...riel.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, mgorman@...e.de,
	jstancek@...hat.com
Subject: Re: [PATCH] sched,numa cap pte scanning overhead to 3% of run time

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.

>  }
>  
>  /*
> @@ -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.
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