Re: [Regression] 3.19-rc3 : memcg: Hang in mount memcg

["Suzuki K. Poulose" <Suzuki.Poulose@arm.com>]

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@gmail.com>
Date:   Wed Dec 10 15:42:28 2014 -0800

     slab: replace smp_read_barrier_depends() with lockless_dereference()



Thanks
Suzuki

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