Re: [PATCH for-4.6] cgroup: ignore css_sets associated with dead cgroups during migration

From: Peter Zijlstra
Date: Wed Mar 16 2016 - 06:46:49 EST

On Tue, Mar 15, 2016 at 08:43:04PM -0400, Tejun Heo wrote:
> Before 2e91fa7f6d45 ("cgroup: keep zombies associated with their
> original cgroups"), all dead tasks were associated with init_css_set.
> If a zombie task is requested for migration, while migration prep
> operations would still be performed on init_css_set, the actual
> migration would ignore zombie tasks. As init_css_set is always valid,
> this worked fine.
> However, after 2e91fa7f6d45, zombie tasks stay with the css_set it was
> associated with at the time of death. Let's say a task T associated
> with cgroup A on hierarchy H-1 and cgroup B on hiearchy H-2. After T
> becomes a zombie, it would still remain associated with A and B. If A
> only contains zombie tasks, it can be removed. On removal, A gets
> marked offline but stays pinned until all zombies are drained. At
> this point, if migration is initiated on T to a cgroup C on hierarchy
> H-2, migration path would try to prepare T's css_set for migration and
> trigger the following.
> WARNING: CPU: 0 PID: 1576 at kernel/cgroup.c:474 cgroup_get+0x121/0x160()
> CPU: 0 PID: 1576 Comm: bash Not tainted 4.4.0-work+ #289
> ...
> Call Trace:
> [<ffffffff8127e63c>] dump_stack+0x4e/0x82
> [<ffffffff810445e8>] warn_slowpath_common+0x78/0xb0
> [<ffffffff810446d5>] warn_slowpath_null+0x15/0x20
> [<ffffffff810c33e1>] cgroup_get+0x121/0x160
> [<ffffffff810c349b>] link_css_set+0x7b/0x90
> [<ffffffff810c4fbc>] find_css_set+0x3bc/0x5e0
> [<ffffffff810c5269>] cgroup_migrate_prepare_dst+0x89/0x1f0
> [<ffffffff810c7547>] cgroup_attach_task+0x157/0x230
> [<ffffffff810c7a17>] __cgroup_procs_write+0x2b7/0x470
> [<ffffffff810c7bdc>] cgroup_tasks_write+0xc/0x10
> [<ffffffff810c4790>] cgroup_file_write+0x30/0x1b0
> [<ffffffff811c68fc>] kernfs_fop_write+0x13c/0x180
> [<ffffffff81151673>] __vfs_write+0x23/0xe0
> [<ffffffff81152494>] vfs_write+0xa4/0x1a0
> [<ffffffff811532d4>] SyS_write+0x44/0xa0
> [<ffffffff814af2d7>] entry_SYSCALL_64_fastpath+0x12/0x6f
> It doesn't make sense to prepare migration for css_sets pointing to
> dead cgroups as they are guaranteed to contain only zombies which are
> ignored later during migration. This patch makes cgroup destruction
> path mark all affected css_sets as dead and updates the migration path
> to ignore them during preparation.
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Fixes: 2e91fa7f6d45 ("cgroup: keep zombies associated with their original cgroups")

This doesn't fix the problem that those zombies might actually still
want to use the cgroups they're tied to, as reported here:

I would strongly suggest to revert 2e91fa7f6d45 wholesale (and mark for
stable) and try again later.