Subject: Re: [PATCH v3 2/2] decrement static keys on real destroy time

From: Tejun Heo <[email-masked]>
Date: Thu, 26 Apr 2012 14:39:16 -0700

Hello, Glauber.

Overall, I like this approach much better.  Just some nits below.

On Thu, Apr 26, 2012 at 06:24:23PM -0300, Glauber Costa wrote:
> @@ -4836,6 +4851,18 @@ static void free_work(struct work_struct *work)
>  	int size = sizeof(struct mem_cgroup);
>  
>  	memcg = container_of(work, struct mem_cgroup, work_freeing);
> +	/*
> +	 * We need to make sure that (at least for now), the jump label
> +	 * destruction code runs outside of the cgroup lock. It is in theory
> +	 * possible to call the cgroup destruction function outside of that
> +	 * lock, but it is not yet done. rate limiting plus the deferred
> +	 * interface for static_branch destruction guarantees that it will
> +	 * run through schedule_work(), therefore, not holding any cgroup
> +	 * related lock (this is, of course, until someone decides to write
> +	 * a schedule_work cgroup :p )
> +	 */

Isn't the above a bit too verbose?  Wouldn't just stating the locking
dependency be enough?

> +	disarm_static_keys(memcg);
>  	if (size < PAGE_SIZE)
>  		kfree(memcg);
>  	else
> diff --git a/net/ipv4/tcp_memcontrol.c b/net/ipv4/tcp_memcontrol.c
> index 1517037..7790008 100644
> --- a/net/ipv4/tcp_memcontrol.c
> +++ b/net/ipv4/tcp_memcontrol.c
> @@ -54,6 +54,8 @@ int tcp_init_cgroup(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
>  	cg_proto->sysctl_mem = tcp->tcp_prot_mem;
>  	cg_proto->memory_allocated = &tcp->tcp_memory_allocated;
>  	cg_proto->sockets_allocated = &tcp->tcp_sockets_allocated;
> +	cg_proto->active = false;
> +	cg_proto->activated = false;

Isn't the memory zallocd?  I find 0 / NULL / false inits unnecessary
and even misleading (can the memory be non-zero here?).  Another side
effect is that it tends to get out of sync as more fields are added.

> +/*
> + * This is to prevent two writes arriving at the same time
> + * at kmem.tcp.limit_in_bytes.
> + *
> + * There is a race at the first time we write to this file:
> + *
> + * - cg_proto->activated == false for all writers.
> + * - They all do a static_key_slow_inc().
> + * - When we are finally read to decrement the static_keys,
                            ^
                            ready

> + *   we'll do it only once per activated cgroup. So we won't
> + *   be able to disable it.
> + *
> + *   Also, after the first caller increments the static_branch
> + *   counter, all others will return right away. That does not mean,
> + *   however, that the update is finished.
> + *
> + *   Without this mutex, it would then be possible for a second writer
> + *   to get to the update site, return 

I kinda don't follow the above sentence.

> + *   When a user updates limit of 2 cgroups at once, following happens.
> + *
> + *   	CPU A				CPU B
> + *
> + *	if (cg_proto->activated)	if (cg->proto_activated)
> + *		static_key_inc()		static_key_inc()
> + * 		=> set counter 0->1		=> set counter 1->2,
> + * 						return immediately.
> + * 		=> hold mutex			=> cg_proto->activated = true. 
> + * 		=> overwrite jmps.

Isn't this something which should be solved from static_keys API?  Why
is this being worked around from memcg?  Also, I again hope that the
explanation is slightly more concise.

Thanks.

-- 
tejun