On 10/01/15 08:55, Vladimir Davydov wrote:
On Fri, Jan 09, 2015 at 05:43:17PM +0000, Suzuki K. Poulose wrote:
Hi
We have hit a hang on ARM64 defconfig, while running LTP tests on
3.19-rc3. We are
in the process of a git bisect and will update the results as and
when we find the commit.
During the ksm ltp run, the test hangs trying to mount memcg with
the following strace
output:
mount("memcg", "/dev/cgroup", "cgroup", 0, "memory") = ?
ERESTARTNOINTR (To be restarted)
mount("memcg", "/dev/cgroup", "cgroup", 0, "memory") = ?
ERESTARTNOINTR (To be restarted)
[ ... repeated forever ... ]
At this point, one can try mounting the memcg to verify the problem.
# mount -t cgroup -o memory memcg memcg_dir
--hangs--
Strangely, if we run the mount command from a cold boot (i.e.
without running LTP first),
then it succeeds.
Upon a quick look we are hitting the following code :
kernel/cgroup.c: cgroup_mount() :
1779 for_each_subsys(ss, i) {
1780 if (!(opts.subsys_mask & (1 << i)) ||
1781 ss->root == &cgrp_dfl_root)
1782 continue;
1783
1784 if
(!percpu_ref_tryget_live(&ss->root->cgrp.self.refcnt)) {
1785 mutex_unlock(&cgroup_mutex);
1786 msleep(10);
1787 ret = restart_syscall(); <=====
1788 goto out_free;
1789 }
1790 cgroup_put(&ss->root->cgrp);
1791 }
with ss->root->cgrp.self.refct.percpu_count_ptr == __PERCPU_REF_ATOMIC_DEAD
Any ideas?
The problem is that the memory cgroup controller takes a css reference
per each charged page and does not reparent charged pages on css
offline, while cgroup_mount/cgroup_kill_sb expect all css references to
offline cgroups to be gone soon, restarting the syscall if the ref count
!= 0. As a result, if you create a memory cgroup, charge some page cache
to it, and then remove it, unmount/mount will hang forever.
May be, we should kill the ref counter to the memory controller root in
cgroup_kill_sb only if there is no children at all, neither online nor
offline.
Still reproducible on 3.19-rc5 with the same setup. From git bisect, the
last good commit is :
commit 8df0c2dcf61781d2efa8e6e5b06870f6c6785735
Author: Pranith Kumar <bobby.prani@xxxxxxxxx>
Date: Wed Dec 10 15:42:28 2014 -0800
slab: replace smp_read_barrier_depends() with lockless_dereference()
Thanks
Suzuki
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