linux-kernel - Re: [PATCH V3] mm/thp: Allocate transparent hugepages on local node

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Date:	Fri, 16 Jan 2015 16:02:04 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	David Rientjes <rientjes@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] mm/thp: Allocate transparent hugepages on local node

On Fri, 16 Jan 2015 12:56:36 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:

> This make sure that we try to allocate hugepages from local node if
> allowed by mempolicy. If we can't, we fallback to small page allocation
> based on mempolicy. This is based on the observation that allocating pages
> on local node is more beneficial than allocating hugepages on remote node.

The changelog is a bit incomplete.  It doesn't describe the current
behaviour, nor what is wrong with it.  What are the before-and-after
effects of this change?

And what might be the user-visible effects?

> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2030,6 +2030,46 @@ retry_cpuset:
>  	return page;
>  }
>  
> +struct page *alloc_hugepage_vma(gfp_t gfp, struct vm_area_struct *vma,
> +				unsigned long addr, int order)

alloc_pages_vma() is nicely documented.  alloc_hugepage_vma() is not
documented at all.  This makes it a bit had for readers to work out the
difference!

Is it possible to scrunch them both into the same function?  Probably
too messy?

> +{
> +	struct page *page;
> +	nodemask_t *nmask;
> +	struct mempolicy *pol;
> +	int node = numa_node_id();
> +	unsigned int cpuset_mems_cookie;
> +
> +retry_cpuset:
> +	pol = get_vma_policy(vma, addr);
> +	cpuset_mems_cookie = read_mems_allowed_begin();
> +
> +	if (pol->mode != MPOL_INTERLEAVE) {
> +		/*
> +		 * For interleave policy, we don't worry about
> +		 * current node. Otherwise if current node is
> +		 * in nodemask, try to allocate hugepage from
> +		 * current node. Don't fall back to other nodes
> +		 * for THP.
> +		 */

This code isn't "interleave policy".  It's everything *but* interleave
policy.  Comment makes no sense!

> +		nmask = policy_nodemask(gfp, pol);
> +		if (!nmask || node_isset(node, *nmask)) {
> +			mpol_cond_put(pol);
> +			page = alloc_pages_exact_node(node, gfp, order);
> +			if (unlikely(!page &&
> +				     read_mems_allowed_retry(cpuset_mems_cookie)))
> +				goto retry_cpuset;
> +			return page;
> +		}
> +	}
> +	mpol_cond_put(pol);
> +	/*
> +	 * if current node is not part of node mask, try
> +	 * the allocation from any node, and we can do retry
> +	 * in that case.
> +	 */
> +	return alloc_pages_vma(gfp, order, vma, addr, node);
> +}

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