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

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Date:	Mon, 19 Jan 2015 17:27:54 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC:	"Kirill A. Shutemov" <kirill@...temov.name>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] mm/thp: Allocate transparent hugepages on local node

On 01/17/2015 01:02 AM, Andrew Morton wrote:
> 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?

Hm that could work, alloc_pages_vma already has an if (MPOL_INTERLEAVE) part, so
just put the THP specialities into an "else if (huge_page)" part there?

You could probably test for GFP_TRANSHUGE the same way as __alloc_pages_slowpath
does. There might be false positives theoretically, but is there anything else
that would use these flags and not be a THP?



>> +{
>> +	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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