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

["Mizuma, Masayoshi" <m.mizuma@jp.fujitsu.com>]

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))
-- 
1.7.1

Thanks,
Masayoshi Mizuma

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