linux-kernel - Re: [RFC PATCH 3/3] mm/mempolicy: implement a partial-interleave mempolicy

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Message-ID: <20231002144035.00000b36@Huawei.com>
Date:   Mon, 2 Oct 2023 14:40:35 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Gregory Price <gourry.memverge@...il.com>
CC:     <linux-mm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <linux-api@...r.kernel.org>,
        <linux-cxl@...r.kernel.org>, <luto@...nel.org>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <dave.hansen@...ux.intel.com>, <hpa@...or.com>, <arnd@...db.de>,
        <akpm@...ux-foundation.org>, <x86@...nel.org>,
        Gregory Price <gregory.price@...verge.com>
Subject: Re: [RFC PATCH 3/3] mm/mempolicy: implement a partial-interleave
 mempolicy

On Thu, 14 Sep 2023 19:54:57 -0400
Gregory Price <gourry.memverge@...il.com> wrote:

> The partial-interleave mempolicy implements interleave on an

I'm not sure 'partial' really conveys what is going on here.
Weighted, or uneven-interleave maybe?

> allocation interval. The default node is the local node, for
> which N pages will be allocated before an interleave pass occurs.
> 
> For example:
>   nodes=0,1,2
>   interval=3
>   cpunode=0
> 
> Over 10 consecutive allocations, the following nodes will be selected:
> [0,0,0,1,2,0,0,0,1,2]
> 
> In this example, there is a 60%/20%/20% distribution of memory.
> 
> Using this mechanism, it becomes possible to define an approximate
> distribution percentage of memory across a set of nodes:
> 
> local_node% : interval/((nr_nodes-1)+interval-1)
> other_node% : (1-local_node%)/(nr_nodes-1)

I'd like to see more discussion here of why you would do this...


A few trivial bits inline,

Jonathan

...

> +static unsigned long alloc_pages_bulk_array_partial_interleave(gfp_t gfp,
> +		struct mempolicy *pol, unsigned long nr_pages,
> +		struct page **page_array)
> +{
> +	nodemask_t nodemask = pol->nodes;
> +	unsigned long nr_pages_main;
> +	unsigned long nr_pages_other;
> +	unsigned long total_cycle;
> +	unsigned long delta;
> +	unsigned long interval;
> +	int allocated = 0;
> +	int start_nid;
> +	int nnodes;
> +	int prev, next;
> +	int i;
> +
> +	/* This stabilizes nodes on the stack incase pol->nodes changes */
> +	barrier();
> +
> +	nnodes = nodes_weight(nodemask);
> +	start_nid = numa_node_id();
> +
> +	if (!node_isset(start_nid, nodemask))
> +		start_nid = first_node(nodemask);
> +
> +	if (nnodes == 1) {
> +		allocated = __alloc_pages_bulk(gfp, start_nid,
> +					       NULL, nr_pages_main,
> +					       NULL, page_array);
> +		return allocated;
		return __alloc_pages_bulk(...)

> +	}
> +	/* We don't want to double-count the main node in calculations */
> +	nnodes--;
> +
> +	interval = pol->part_int.interval;
> +	total_cycle = (interval + nnodes);

excess brackets. Same in various other places.


> +	/* Number of pages on main node: (cycles*interval + up to interval) */
> +	nr_pages_main = ((nr_pages / total_cycle) * interval);
> +	nr_pages_main += (nr_pages % total_cycle % (interval + 1));


> +	/* Number of pages on others: (remaining/nodes) + 1 page if delta  */
> +	nr_pages_other = (nr_pages - nr_pages_main) / nnodes;
> +	nr_pages_other /= nnodes;
> +	/* Delta is number of pages beyond interval up to full cycle */
> +	delta = nr_pages - (nr_pages_main + (nr_pages_other * nnodes));
> +
> +	/* start by allocating for the main node, then interleave rest */
> +	prev = start_nid;
> +	allocated = __alloc_pages_bulk(gfp, start_nid, NULL, nr_pages_main,
> +				       NULL, page_array);
> +	for (i = 0; i < nnodes; i++) {
> +		int pages = nr_pages_other + (delta-- ? 1 : 0);
> +
> +		next = next_node_in(prev, nodemask);
> +		if (next < MAX_NUMNODES)
> +			prev = next;
> +		allocated += __alloc_pages_bulk(gfp, next, NULL, pages,
> +						NULL, page_array);
> +	}
> +
> +	return allocated;
> +}
> +