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

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

On Fri, Jan 09, 2015 at 09:46:49PM +0000, Tejun Heo wrote:
> On Fri, Jan 09, 2015 at 05:43:17PM +0000, Suzuki K. Poulose wrote:
> > 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.
>
> I don't know what LTP is doing and this could actually be hitting on
> an actual bug but if it's trying to move memcg back from unified
> hierarchy to an old one, that might hang - it should prolly made to
> just fail at that point.  Anyways, any chance you can find out what
> happened, in terms of cgroup mounting, to memcg upto that point?
>

This is what the test(ksm03) does, roughly from strace :

faccessat(AT_FDCWD, "/sys/kernel/mm/ksm/", F_OK) = 0
faccessat(AT_FDCWD, "/sys/kernel/mm/ksm/merge_across_nodes", F_OK) = -1 ENOENT (No such file or directory)
mkdirat(AT_FDCWD, "/dev/cgroup", 0777)  = 0
mount("memcg", "/dev/cgroup", "cgroup", 0, "memory") = 0

--- set memory limit. Create a new set /dev/cgroups/1 and moves test to that group ---
mkdirat(AT_FDCWD, "/dev/cgroup/1", 0777) = 0
openat(AT_FDCWD, "/dev/cgroup/1/memory.limit_in_bytes", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
fstat(3, {st_dev=makedev(0, 24), st_ino=41, st_mode=S_IFREG|0644, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0, st_size=0, st_atime=2015/01/12-15:10:13, st_mtime=2015/01/12-15:10:13, st_ctime=2015/01/12-15:10:13}) = 0
mmap(NULL, 65536, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb2903000
write(3, "1073741824", 10)              = 10
close(3)                                = 0
munmap(0x7fb2903000, 65536)             = 0
getpid()                                = 1324
openat(AT_FDCWD, "/dev/cgroup/1/tasks", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
fstat(3, {st_dev=makedev(0, 24), st_ino=37, st_mode=S_IFREG|0644, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0, st_size=0, st_atime=2015/01/12-15:10:13, st_mtime=2015/01/12-15:10:13, st_ctime=2015/01/12-15:10:13}) = 0
mmap(NULL, 65536, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb2903000
write(3, "1324", 4)                     = 4
close(3)                                = 0
munmap(0x7fb2903000, 65536)             = 0

clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fb2a7f0d0) = 1325
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fb2a7f0d0) = 1326
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fb2a7f0d0) = 1327

--- Creates 3 children, perform a lot of memory operations with shared pages
    verify the ksm for activity and wait for children to exit ---

wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 1325
wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 1326
wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 1327
wait4(-1, 0x7fe5625f3c, WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes)

--- cleanup: Move tasks under /dev/cgroups/1/ to /dev/cgroups/ and delete subdir, umount cgroup ---

faccessat(AT_FDCWD, "/sys/kernel/mm/ksm/merge_across_nodes", F_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/dev/cgroup/tasks", O_WRONLY) = 205
openat(AT_FDCWD, "/dev/cgroup/1/tasks", O_RDONLY) = 206
fstat(206, {st_dev=makedev(0, 24), st_ino=37, st_mode=S_IFREG|0644, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0, st_size=0, st_atime=2015/01/12-15:10:13, st_mtime=2015/01/12-15:10:13, st_ctime=2015/01/12-15:10:13}) = 0
mmap(NULL, 65536, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb1c53000
read(206, "1324\n", 4096)               = 5
write(205, "1324", 4)                   = 4
read(206, "", 4096)                     = 0
close(205)                              = 0
close(206)                              = 0
munmap(0x7fb1c53000, 65536)             = 0
unlinkat(AT_FDCWD, "/dev/cgroup/1", AT_REMOVEDIR) = 0
umount2("/dev/cgroup", 0)               = 0
unlinkat(AT_FDCWD, "/dev/cgroup", AT_REMOVEDIR) = 0
exit_group(0)                           = ?


The next invocation of the same test fails to mount the cgroup memory.

Thanks
Suzuki

> Thanks.
>
> --
> tejun
>

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