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

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Date:	Fri, 16 Jan 2015 23:15:59 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"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, 2015-01-16 at 16:02 -0800, 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?

I'd be interested in any performance data. I'll run this by a 4 node box
next week.

> 
> > --- 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!

May I add that, while a nit, this indentation is quite ugly:

> 
> > +		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;
> > +		}
> > +	}

Improving it makes the code visually easier on the eye. So this should
be considered if another re-spin of the patch is to be done anyway. Just
jump to the mpol refcounting and be done when 'pol->mode ==
MPOL_INTERLEAVE'.

Thanks,
Davidlohr

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