linux-kernel - Re: [RFC PATCH] mm/hugetlb: Avoid soft lockup in set_max_huge

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Date:	Tue, 26 Jul 2016 08:58:50 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Jia He <hejianet@...il.com>, linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Michal Hocko <mhocko@...e.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [RFC PATCH] mm/hugetlb: Avoid soft lockup in set_max_huge_pages()

On 07/26/2016 08:44 AM, Jia He wrote:
> This patch is to fix such soft lockup. I thouhgt it is safe to call 
> cond_resched() because alloc_fresh_gigantic_page and alloc_fresh_huge_page 
> are out of spin_lock/unlock section.

Yikes.  So the call site for both the things you patch is this:

>         while (count > persistent_huge_pages(h)) {
...
>                 spin_unlock(&hugetlb_lock);
>                 if (hstate_is_gigantic(h))
>                         ret = alloc_fresh_gigantic_page(h, nodes_allowed);
>                 else
>                         ret = alloc_fresh_huge_page(h, nodes_allowed);
>                 spin_lock(&hugetlb_lock);

and you choose to patch both of the alloc_*() functions.  Why not just
fix it at the common call site?  Seems like that
spin_lock(&hugetlb_lock) could be a cond_resched_lock() which would fix
both cases.

Also, putting that cond_resched() inside the for_each_node*() loop is an
odd choice.  It seems to indicate that the loops can take a long time,
which really isn't the case.  The _loop_ isn't long, right?