Re: [RFC][PATCH] cgroup_threadgroup_rwsem - affects scalability and OOM

[Balbir Singh <bsingharora@gmail.com>]

On Tue, Aug 09, 2016 at 05:00:59PM +0800, Zefan Li wrote:
> > This almost stalls the system, this patch moves the threadgroup_change_begin
> > from before cgroup_fork() to just before cgroup_canfork(). Ideally we shouldn't
> > have 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.
> > 
> > There are other theoretical issues with this semaphore
> > 
> > systemd can do
> > 
> > 1. cgroup_mutex (cgroup_kn_lock_live)
> > 2. cgroup_threadgroup_rwsem (W) (__cgroup_procs_write)
> > 
> > and other threads can go
> > 
> > 1. cgroup_threadgroup_rwsem (R) (copy_process)
> > 2. mem_cgroup_iter (as a part of reclaim) (cgroup_mutex -- rcu lock or cgroup_mutex)
> > 
> > However, I've not examined them in too much detail or looked at lockdep
> > wait chains for those paths.
> > 
> > I am sure there is a good reason for placing cgroup_threadgroup_rwsem
> > where it is today and I might be missing something. I am also surprised
> > no-one else has run into it so far.
> > 
> > Comments?
> > 
> 
> We used to use cgroup_threadgroup_rwsem for syncronization between threads
> in the same threadgroup, but now it has evolved to ensure atomic operations
> across multi processes.
> 

Yes and it seems incorrect

> For example, I'm trying to fix a race. See https://lkml.org/lkml/2016/8/8/900
> 
> And the fix kind of relies on the fact that cgroup_post_fork() is placed
> inside the read section of cgroup_threadgroup_rwsem, so that cpuset_fork()
> won't race with cgroup migration.
>

My patch retains that behaviour, before ss->fork() is called we hold
the cgroup_threadgroup_rwsem, in fact it is held prior to ss->can_fork()


Balbir 

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