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Subject: Re: [RFC PATCH] mm/hugetlb: Avoid soft lockup in set_max_huge_pages()
To: Jia He <hejianet@gmail.com>, linux-mm@kvack.org
References: <1469547868-9814-1-git-send-email-hejianet@gmail.com>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Michal Hocko <mhocko@suse.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <579788BA.1040706@linux.intel.com>
Date: Tue, 26 Jul 2016 08:58:50 -0700
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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?