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cgroup: reduce read locked section of cgroup_threadgroup_rwsem during fork 

cgroup_threadgroup_rwsem is acquired in read mode during process exit
and fork.  It is also grabbed in write mode during
__cgroups_proc_write().  I've recently run into a scenario with lots
of memory pressure and OOM and I am beginning to see

systemd

 __switch_to+0x1f8/0x350
 __schedule+0x30c/0x990
 schedule+0x48/0xc0
 percpu_down_write+0x114/0x170
 __cgroup_procs_write.isra.12+0xb8/0x3c0
 cgroup_file_write+0x74/0x1a0
 kernfs_fop_write+0x188/0x200
 __vfs_write+0x6c/0xe0
 vfs_write+0xc0/0x230
 SyS_write+0x6c/0x110
 system_call+0x38/0xb4

This thread is waiting on the reader of cgroup_threadgroup_rwsem to
exit.  The reader itself is under memory pressure and has gone into
reclaim after fork. There are times the reader also ends up waiting on
oom_lock as well.

 __switch_to+0x1f8/0x350
 __schedule+0x30c/0x990
 schedule+0x48/0xc0
 jbd2_log_wait_commit+0xd4/0x180
 ext4_evict_inode+0x88/0x5c0
 evict+0xf8/0x2a0
 dispose_list+0x50/0x80
 prune_icache_sb+0x6c/0x90
 super_cache_scan+0x190/0x210
 shrink_slab.part.15+0x22c/0x4c0
 shrink_zone+0x288/0x3c0
 do_try_to_free_pages+0x1dc/0x590
 try_to_free_pages+0xdc/0x260
 __alloc_pages_nodemask+0x72c/0xc90
 alloc_pages_current+0xb4/0x1a0
 page_table_alloc+0xc0/0x170
 __pte_alloc+0x58/0x1f0
 copy_page_range+0x4ec/0x950
 copy_process.isra.5+0x15a0/0x1870
 _do_fork+0xa8/0x4b0
 ppc_clone+0x8/0xc

In the meanwhile, all processes exiting/forking are blocked almost
stalling the system.

This patch moves the threadgroup_change_begin from before
cgroup_fork() to just before cgroup_canfork().  There is no nee to
worry about threadgroup changes till the task is actually added to the
threadgroup.  This avoids having to call reclaim with
cgroup_threadgroup_rwsem held.

tj: Subject and description edits.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Balbir Singh 2016-08-10 15:43:06 -04:00 committed by Tejun Heo
parent 06f4e94898
commit 568ac88821
1 changed files with 2 additions and 2 deletions
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4
kernel/fork.c
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@ -1404,7 +1404,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
p->real_start_time = ktime_get_boot_ns();
p->io_context = NULL;
p->audit_context = NULL;
threadgroup_change_begin(current);
cgroup_fork(p);
#ifdef CONFIG_NUMA
p->mempolicy = mpol_dup(p->mempolicy);
@ -1556,6 +1555,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
INIT_LIST_HEAD(&p->thread_group);
p->task_works = NULL;
threadgroup_change_begin(current);
/*
* Ensure that the cgroup subsystem policies allow the new process to be
* forked. It should be noted the the new process's css_set can be changed
@ -1656,6 +1656,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
bad_fork_cancel_cgroup:
cgroup_cancel_fork(p);
bad_fork_free_pid:
threadgroup_change_end(current);
if (pid != &init_struct_pid)
free_pid(pid);
bad_fork_cleanup_thread:
@ -1688,7 +1689,6 @@ bad_fork_cleanup_policy:
mpol_put(p->mempolicy);
bad_fork_cleanup_threadgroup_lock:
#endif
threadgroup_change_end(current);
delayacct_tsk_free(p);
bad_fork_cleanup_count:
atomic_dec(&p->cred->user->processes);