linux-kernel - Re: [PATCH 4/8] sched: Update NUMA hinting faults once per scan

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Date:	Fri, 28 Jun 2013 12:02:33 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] sched: Update NUMA hinting faults once per scan

* Mel Gorman <mgorman@...e.de> [2013-06-26 15:38:03]:

> NUMA hinting faults counts and placement decisions are both recorded in the
> same array which distorts the samples in an unpredictable fashion. The values
> linearly accumulate during the scan and then decay creating a sawtooth-like
> pattern in the per-node counts. It also means that placement decisions are
> time sensitive. At best it means that it is very difficult to state that
> the buffer holds a decaying average of past faulting behaviour. At worst,
> it can confuse the load balancer if it sees one node with an artifically high
> count due to very recent faulting activity and may create a bouncing effect.
> 
> This patch adds a second array. numa_faults stores the historical data
> which is used for placement decisions. numa_faults_buffer holds the
> fault activity during the current scan window. When the scan completes,
> numa_faults decays and the values from numa_faults_buffer are copied
> across.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
>  include/linux/sched.h | 13 +++++++++++++
>  kernel/sched/core.c   |  1 +
>  kernel/sched/fair.c   | 16 +++++++++++++---
>  3 files changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index ba46a64..42f9818 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1506,7 +1506,20 @@ struct task_struct {
>  	u64 node_stamp;			/* migration stamp  */
>  	struct callback_head numa_work;
>  
> +	/*
> +	 * Exponential decaying average of faults on a per-node basis.
> +	 * Scheduling placement decisions are made based on the these counts.
> +	 * The values remain static for the duration of a PTE scan
> +	 */
>  	unsigned long *numa_faults;
> +
> +	/*
> +	 * numa_faults_buffer records faults per node during the current
> +	 * scan window. When the scan completes, the counts in numa_faults
> +	 * decay and these values are copied.
> +	 */
> +	unsigned long *numa_faults_buffer;
> +
>  	int numa_preferred_nid;
>  #endif /* CONFIG_NUMA_BALANCING */
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 019baae..b00b81a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1596,6 +1596,7 @@ static void __sched_fork(struct task_struct *p)
>  	p->numa_preferred_nid = -1;
>  	p->numa_work.next = &p->numa_work;
>  	p->numa_faults = NULL;
> +	p->numa_faults_buffer = NULL;
>  #endif /* CONFIG_NUMA_BALANCING */
>  }
>  
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f8c3f61..5893399 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -805,8 +805,14 @@ static void task_numa_placement(struct task_struct *p)
>  
>  	/* Find the node with the highest number of faults */
>  	for (nid = 0; nid < nr_node_ids; nid++) {
> -		unsigned long faults = p->numa_faults[nid];
> +		unsigned long faults;
> +
> +		/* Decay existing window and copy faults since last scan */
>  		p->numa_faults[nid] >>= 1;
> +		p->numa_faults[nid] += p->numa_faults_buffer[nid];
> +		p->numa_faults_buffer[nid] = 0;
> +
> +		faults = p->numa_faults[nid];
>  		if (faults > max_faults) {
>  			max_faults = faults;
>  			max_nid = nid;
> @@ -831,9 +837,13 @@ void task_numa_fault(int node, int pages, bool migrated)
>  	if (unlikely(!p->numa_faults)) {
>  		int size = sizeof(*p->numa_faults) * nr_node_ids;
>  
> -		p->numa_faults = kzalloc(size, GFP_KERNEL);
> +		/* numa_faults and numa_faults_buffer share the allocation */
> +		p->numa_faults = kzalloc(size * 2, GFP_KERNEL);

Instead of allocating buffer to hold the current faults, cant we pass
the nr of pages and node information (and probably migrate) to
task_numa_placement()?.

Why should task_struct be passed as an argument to  task_numa_placement().
It seems it always will be current.

>  		if (!p->numa_faults)
>  			return;
> +
> +		BUG_ON(p->numa_faults_buffer);
> +		p->numa_faults_buffer = p->numa_faults + nr_node_ids;
>  	}
>  
>  	/*
> @@ -847,7 +857,7 @@ void task_numa_fault(int node, int pages, bool migrated)
>  	task_numa_placement(p);
>  
>  	/* Record the fault, double the weight if pages were migrated */
> -	p->numa_faults[node] += pages << migrated;
> +	p->numa_faults_buffer[node] += pages << migrated;
>  }
>  
>  static void reset_ptenuma_scan(struct task_struct *p)
> -- 
> 1.8.1.4
> 
> --
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-- 
Thanks and Regards
Srikar Dronamraju

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