Re: [RFC PATCH] mm/thp: Always allocate transparent hugepages on local node

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

David Rientjes <rientjes@google.com> writes:

> On Mon, 24 Nov 2014, Kirill A. Shutemov wrote:
>
>> > This make sure that we try to allocate hugepages from local node. 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 that allocating hugepages on remote node.
>> 
>> Local node on allocation is not necessary local node for use.
>> If policy says to use a specific node[s], we should follow.
>> 
>
> True, and the interaction between thp and mempolicies is fragile: if a 
> process has a MPOL_BIND mempolicy over a set of nodes, that does not 
> necessarily mean that we want to allocate thp remotely if it will always 
> be accessed remotely.  It's simple to benchmark and show that remote 
> access latency of a hugepage can exceed that of local pages.  MPOL_BIND 
> itself is a policy of exclusion, not inclusion, and it's difficult to 
> define when local pages and its cost of allocation is better than remote 
> thp.
>
> For MPOL_BIND, if the local node is allowed then thp should be forced from 
> that node, if the local node is disallowed then allocate from any node in 
> the nodemask.  For MPOL_INTERLEAVE, I think we should only allocate thp 
> from the next node in order, otherwise fail the allocation and fallback to 
> small pages.  Is this what you meant as well?
>

Something like below

struct page *alloc_hugepage_vma(gfp_t gfp, struct vm_area_struct *vma,
				unsigned long addr, int order)
{
	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 (unlikely(pol->mode == MPOL_INTERLEAVE)) {
		unsigned nid;
		nid = interleave_nid(pol, vma, addr, PAGE_SHIFT + order);
		mpol_cond_put(pol);
		page = alloc_page_interleave(gfp, order, nid);
		if (unlikely(!page &&
			     read_mems_allowed_retry(cpuset_mems_cookie)))
			goto retry_cpuset;
		return page;
	}
	nmask = policy_nodemask(gfp, pol);
	if (!nmask || node_isset(node, *nmask)) {
		mpol_cond_put(pol);
		page = alloc_hugepage_exact_node(node, gfp, order);
		if (unlikely(!page &&
			     read_mems_allowed_retry(cpuset_mems_cookie)))
			goto retry_cpuset;
		return page;

	}
	/*
	 * if current node is not part of node mask, try
	 * the allocation from any node, and we can do retry
	 * in that case.
	 */
	page = __alloc_pages_nodemask(gfp, order,
				      policy_zonelist(gfp, pol, node),
				      nmask);
	mpol_cond_put(pol);
	if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie)))
		goto retry_cpuset;

	return page;
}

-aneesh

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