Re: [PATCH v2 1/1] mm: hugetlb: fix stalling when a large number of hugepages are freed

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Apr 07, 2014 at 05:24:03PM +0900, Masayoshi Mizuma wrote:
> When I decrease the value of nr_hugepage in procfs a lot, a long stalling
> 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 the long stalling 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 the long stalling.
> 
> 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 a long stalling (about 20sec),
> in this decreasing rate.
> 
> * Changes in v2
> - Adding cond_resched_lock() in return_unused_surplus_pages()
>   Because when freeing a number of surplus pages, same problems happen.
> 
> 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>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks,
Naoya Horiguchi

> ---
>  mm/hugetlb.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 7d57af2..761ef5b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1160,6 +1160,7 @@ static void return_unused_surplus_pages(struct hstate *h,
>  	while (nr_pages--) {
>  		if (!free_pool_huge_page(h, &node_states[N_MEMORY], 1))
>  			break;
> +		cond_resched_lock(&hugetlb_lock);
>  	}
>  }
>  
> @@ -1535,6 +1536,7 @@ 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))
> -- 
> 1.7.1
> 

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