linux-kernel - Re: [PATCH] hugetlb: cond_resched for set_max_huge_pages and follow_hugetlb

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Date:	Thu, 23 Jul 2015 15:08:58 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Spencer Baugh <sbaugh@...ern.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	open list <linux-kernel@...r.kernel.org>,
	Joern Engel <joern@...estorage.com>,
	Spencer Baugh <Spencer.baugh@...estorage.com>,
	Joern Engel <joern@...fs.org>
Subject: Re: [PATCH] hugetlb: cond_resched for set_max_huge_pages and
 follow_hugetlb_page

On Thu, 23 Jul 2015, Spencer Baugh wrote:

> From: Joern Engel <joern@...fs.org>
> 
> ~150ms scheduler latency for both observed in the wild.
> 
> Signed-off-by: Joern Engel <joern@...fs.org>
> Signed-off-by: Spencer Baugh <sbaugh@...ern.com>
> ---
>  mm/hugetlb.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a8c3087..2eb6919 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1836,6 +1836,7 @@ static unsigned long set_max_huge_pages(struct hstate *h, unsigned long count,
>  			ret = alloc_fresh_gigantic_page(h, nodes_allowed);
>  		else
>  			ret = alloc_fresh_huge_page(h, nodes_allowed);
> +		cond_resched();
>  		spin_lock(&hugetlb_lock);
>  		if (!ret)
>  			goto out;

This is wrong, you'd want to do any cond_resched() before the page 
allocation to avoid racing with an update to h->nr_huge_pages or 
h->surplus_huge_pages while hugetlb_lock was dropped that would result in 
the page having been uselessly allocated.

> @@ -3521,6 +3522,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  				spin_unlock(ptl);
>  			ret = hugetlb_fault(mm, vma, vaddr,
>  				(flags & FOLL_WRITE) ? FAULT_FLAG_WRITE : 0);
> +			cond_resched();
>  			if (!(ret & VM_FAULT_ERROR))
>  				continue;
>  

This is almost certainly the wrong placement as well since it's inserted 
inside a conditional inside a while loop and there's no reason to 
hugetlb_fault(), schedule, and then check the return value.  You need to 
insert your cond_resched()'s in legitimate places.
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