Patch "cgroup: reduce read locked section of cgroup_threadgroup_rwsem during fork" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

cgroup: reduce read locked section of cgroup_threadgroup_rwsem during fork

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
cgroup-reduce-read-locked-section-of-cgroup_threadgroup_rwsem-during-fork.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Balbir Singh <[email protected]>
Date: Wed, 10 Aug 2016 15:43:06 -0400
Subject: cgroup: reduce read locked section of cgroup_threadgroup_rwsem during fork

From: Balbir Singh <[email protected]>

commit 568ac888215c7fb2fabe8ea739b00ec3c1f5d440 upstream.

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.

---
 kernel/fork.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -1406,7 +1406,6 @@ static struct task_struct *copy_process(
 	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);

@@ -1558,6 +1557,7 @@ static struct task_struct *copy_process(
 	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

@@ -1658,6 +1658,7 @@ static struct task_struct *copy_process(
 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:

@@ -1690,7 +1691,6 @@ 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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/cgroup-reduce-read-locked-section-of-cgroup_threadgroup_rwsem-during-fork.patch