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Date: Wed, 17 Jan 2024 23:06:46 -0500
From: Gregory Price <gregory.price@memverge.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Gregory Price <gourry.memverge@gmail.com>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
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	hannes@cmpxchg.org, dan.j.williams@intel.com,
	Srinivasulu Thanneeru <sthanneeru.opensrc@micron.com>
Subject: Re: [PATCH 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE for
 weighted interleaving
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References: <20240112210834.8035-1-gregory.price@memverge.com>
 <20240112210834.8035-4-gregory.price@memverge.com>
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On Thu, Jan 18, 2024 at 11:05:52AM +0800, Huang, Ying wrote:
> Gregory Price <gourry.memverge@gmail.com> writes:
> > +static unsigned long alloc_pages_bulk_array_weighted_interleave(gfp_t gfp,
> > +		struct mempolicy *pol, unsigned long nr_pages,
> > +		struct page **page_array)
> > +{
> > +	struct task_struct *me = current;
> > +	unsigned long total_allocated = 0;
> > +	unsigned long nr_allocated;
> > +	unsigned long rounds;
> > +	unsigned long node_pages, delta;
> > +	u8 weight;
> > +	struct iw_table __rcu *table;
> > +	u8 *weights;
> > +	unsigned int weight_total = 0;
> > +	unsigned long rem_pages = nr_pages;
> > +	nodemask_t nodes;
> > +	int nnodes, node, weight_nodes;
> > +	int prev_node = NUMA_NO_NODE;
> > +	int i;
> > +
> > +	nnodes = read_once_policy_nodemask(pol, &nodes);
> > +	if (!nnodes)
> > +		return 0;
> > +
> > +	/* Continue allocating from most recent node and adjust the nr_pages */
> > +	if (pol->wil.cur_weight) {
> > +		node = next_node_in(me->il_prev, nodes);
> > +		node_pages = pol->wil.cur_weight;
> > +		if (node_pages > rem_pages)
> > +			node_pages = rem_pages;
> > +		nr_allocated = __alloc_pages_bulk(gfp, node, NULL, node_pages,
> > +						  NULL, page_array);
> > +		page_array += nr_allocated;
> > +		total_allocated += nr_allocated;
> > +		/* if that's all the pages, no need to interleave */
> > +		if (rem_pages <= pol->wil.cur_weight) {
> > +			pol->wil.cur_weight -= rem_pages;
> > +			return total_allocated;
> > +		}
> > +		/* Otherwise we adjust nr_pages down, and continue from there */
> > +		rem_pages -= pol->wil.cur_weight;
> > +		pol->wil.cur_weight = 0;
> > +		prev_node = node;
> > +	}
> > +
> > +	/* fetch the weights for this operation and calculate total weight */
> > +	weights = kmalloc(nnodes, GFP_KERNEL);
> > +	if (!weights)
> > +		return total_allocated;
> > +
> > +	rcu_read_lock();
> > +	table = rcu_dereference(iw_table);
> > +	weight_nodes = 0;
> > +	for_each_node_mask(node, nodes) {
> > +		weights[weight_nodes++] = table->weights[node];
> > +		weight_total += table->weights[node];
> > +	}
> > +	rcu_read_unlock();
> > +
> > +	if (!weight_total) {
> > +		kfree(weights);
> > +		return total_allocated;
> > +	}
> > +
> > +	/* Now we can continue allocating as if from 0 instead of an offset */
> > +	rounds = rem_pages / weight_total;
> > +	delta = rem_pages % weight_total;
> > +	for (i = 0; i < nnodes; i++) {
> > +		node = next_node_in(prev_node, nodes);
> > +		weight = weights[i];
> > +		node_pages = weight * rounds;
> > +		if (delta) {
> > +			if (delta > weight) {
> > +				node_pages += weight;
> > +				delta -= weight;
> > +			} else {
> > +				node_pages += delta;
> > +				delta = 0;
> > +			}
> > +		}
> > +		nr_allocated = __alloc_pages_bulk(gfp, node, NULL, node_pages,
> > +						  NULL, page_array);
> > +		page_array += nr_allocated;
> > +		total_allocated += nr_allocated;
> > +		if (total_allocated == nr_pages)
> > +			break;
> > +		prev_node = node;
> > +	}
> > +
> > +	/*
> > +	 * Finally, we need to update me->il_prev and pol->wil.cur_weight
> > +	 * if there were overflow pages, but not equivalent to the node
> > +	 * weight, set the cur_weight to node_weight - delta and the
> > +	 * me->il_prev to the previous node. Otherwise if it was perfect
> > +	 * we can simply set il_prev to node and cur_weight to 0
> > +	 */
> > +	if (node_pages) {
> > +		me->il_prev = prev_node;
> > +		node_pages %= weight;
> > +		pol->wil.cur_weight = weight - node_pages;
> > +	} else {
> > +		me->il_prev = node;
> > +		pol->wil.cur_weight = 0;
> > +	}
> 
> 
> It appears that we should set me->il_prev and pol->wil.cur_weight when
> delta becomes 0?  That is, following allocation should start from there?
> 

So the observation is that when delta reaches 0, we know what the prior
node should be.  The only corner case being that delta is 0 when we
enter the loop (in which case current prev_node is the correct
prev_node).

Eyeballing it, this seems correct, but I'll do some additional
validation tomorrow. That should clean up the last block a bit.

Thanks!
~Gregory