Re: [PATCH] mm: hugetlb: fix softlockup when a large number of hugepages are freed.

Naoya Horiguchi
2014-03-31T15:02:31Z

On Mon, Mar 31, 2014 at 07:43:32PM +0900, Mizuma, Masayoshi wrote:
> Hi,
>
> When I decrease the value of nr_hugepage in procfs a lot, softlockup happens.
> It is because there is no chance of context switch during this process.
>
> On the other hand, when I allocate a large number of hugepages,
> there is some chance of context switch. Hence softlockup doesn't happen
> during this process. So it's necessary to add the context switch
> in the freeing process as same as allocating process to avoid softlockup.
>
> When I freed 12 TB hugapages with kernel-2.6.32-358.el6, the freeing process
> occupied a CPU over 150 seconds and following softlockup message appeared
> twice or more.
>
> --
> $ echo 6000000 > /proc/sys/vm/nr_hugepages
> $ cat /proc/sys/vm/nr_hugepages
> 6000000
> $ grep ^Huge /proc/meminfo
> HugePages_Total: 6000000
> HugePages_Free: 6000000
> HugePages_Rsvd: 0
> HugePages_Surp: 0
> Hugepagesize: 2048 kB
> $ echo 0 > /proc/sys/vm/nr_hugepages
>
> BUG: soft lockup - CPU#16 stuck for 67s! [sh:12883] ...
> Pid: 12883, comm: sh Not tainted 2.6.32-358.el6.x86_64 #1
> Call Trace:
> [<ffffffff8115a438>] ? free_pool_huge_page+0xb8/0xd0
> [<ffffffff8115a578>] ? set_max_huge_pages+0x128/0x190
> [<ffffffff8115c663>] ? hugetlb_sysctl_handler_common+0x113/0x140
> [<ffffffff8115c6de>] ? hugetlb_sysctl_handler+0x1e/0x20
> [<ffffffff811f3097>] ? proc_sys_call_handler+0x97/0xd0
> [<ffffffff811f30e4>] ? proc_sys_write+0x14/0x20
> [<ffffffff81180f98>] ? vfs_write+0xb8/0x1a0
> [<ffffffff81181891>] ? sys_write+0x51/0x90
> [<ffffffff810dc565>] ? __audit_syscall_exit+0x265/0x290
> [<ffffffff8100b072>] ? system_call_fastpath+0x16/0x1b
> --
> I have not confirmed this problem with upstream kernels because I am not
> able to prepare the machine equipped with 12TB memory now.
> However I confirmed that the amount of decreasing hugepages was directly
> proportional to the amount of required time.
>
> I measured required times on a smaller machine. It showed 130-145 hugepages
> decreased in a millisecond.
>
> Amount of decreasing Required time Decreasing rate
> hugepages (msec) (pages/msec)
> ------------------------------------------------------------
> 10,000 pages == 20GB 70 - 74 135-142
> 30,000 pages == 60GB 208 - 229 131-144
>
> It means decrement of 6TB hugepages will trigger softlockup with the default
> threshold 20sec, in this decreasing rate.
>
> Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
> Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> ---
> mm/hugetlb.c | 1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 7d57af2..fe67f2c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1535,6 +1535,7 @@ static unsigned long set_max_huge_pages(struct hstate *h, unsigned long count,
> while (min_count < persistent_huge_pages(h)) {
> if (!free_pool_huge_page(h, nodes_allowed, 0))
> break;
> + cond_resched_lock(&hugetlb_lock);
> }
> while (count < persistent_huge_pages(h)) {
> if (!adjust_pool_surplus(h, nodes_allowed, 1))

It seems that the same thing could happen when freeing a number of surplus pages,
so how about adding cond_resched_lock() also in return_unused_surplus_pages()?

Thanks,
Naoya Horiguchi