linux-kernel - Re: [PATCH 32/52] sched: Track groups of shared tasks

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Date:	Mon, 03 Dec 2012 17:46:17 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 32/52] sched: Track groups of shared tasks

On 12/02/2012 01:43 PM, Ingo Molnar wrote:

> This is not entirely correct as this task might have scheduled or
> migrate ther - but statistically there will be correlation to the
           ^^^^ there?

> tasks that we share memory with, and correlation is all we need.
>
> We map out the relation itself by filtering out the highest address
> ask that is below our own task address, per working set scan
   ^^^ task?
> iteration.

> @@ -906,23 +945,122 @@ out_backoff:
>   }
>
>   /*
> + * Track our "memory buddies" the tasks we actively share memory with.
> + *
> + * Firstly we establish the identity of some other task that we are
> + * sharing memory with by looking at rq[page::last_cpu].curr - i.e.
> + * we check the task that is running on that CPU right now.
> + *
> + * This is not entirely correct as this task might have scheduled or
> + * migrate ther - but statistically there will be correlation to the
               ^^^^ there

> + * tasks that we share memory with, and correlation is all we need.
> + *
> + * We map out the relation itself by filtering out the highest address
> + * ask that is below our own task address, per working set scan
       ^^^ task?

If that word is "task", the comment makes sense. If it is
something else, I'm back to square one on what the code does :)


>   void task_numa_fault(int node, int last_cpu, int pages)
>   {
>   	struct task_struct *p = current;
>   	int priv = (task_cpu(p) == last_cpu);
> +	int idx = 2*node + priv;
>
>   	if (unlikely(!p->numa_faults)) {
> -		int size = sizeof(*p->numa_faults) * 2 * nr_node_ids;
> +		int entries = 2*nr_node_ids;
> +		int size = sizeof(*p->numa_faults) * entries;
>
> -		p->numa_faults = kzalloc(size, GFP_KERNEL);
> +		p->numa_faults = kzalloc(2*size, GFP_KERNEL);

So we multiply nr_node_ids by 2. Twice.

That kind of magic deserves a comment explaining how
and why.  How about:

	/*
	 * We track two arrays with private and shared faults
	 * for each NUMA node. The p->numa_faults_curr array
	 * is allocated at the same time as the p->numa_faults
	 * array.
	 */
	int size = sizeof(*p->numa_faults) * 4 * nr_node_ids;

>   		if (!p->numa_faults)
>   			return;
> +		/*
> +		 * For efficiency reasons we allocate ->numa_faults[]
> +		 * and ->numa_faults_curr[] at once and split the
> +		 * buffer we get. They are separate otherwise.
> +		 */
> +		p->numa_faults_curr = p->numa_faults + entries;
>   	}


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