Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jul 14, 2016 at 09:20:18PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 14, 2016 at 08:43:51PM +0200, Oleg Nesterov wrote:
> > 
> > Oh well... I personally do not think this is what we want... Can't
> > we just add the stupid rcu_sync_enter() into cgroup_init() at least
> > for now? Yes, this means the unnecessary .sync() at boot time, but
> > it will go away after cleanups I am going to send.
> 
> Something like so then?
> 
> ---
> --- a/include/linux/rcu_sync.h
> +++ b/include/linux/rcu_sync.h
> @@ -59,6 +59,7 @@ static inline bool rcu_sync_is_idle(stru
>  }
> 
>  extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
> +extern void rcu_sync_sabotage(struct rcu_sync *);
>  extern void rcu_sync_enter(struct rcu_sync *);
>  extern void rcu_sync_exit(struct rcu_sync *);
>  extern void rcu_sync_dtor(struct rcu_sync *);
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -5609,6 +5609,8 @@ int __init cgroup_init(void)
>  	BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
>  	BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));
> 
> +	rcu_sync_sabotage(&cgroup_threadgroup_rwsem.rss);

With a name like that...

> +
>  	get_user_ns(init_cgroup_ns.user_ns);
> 
>  	mutex_lock(&cgroup_mutex);
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -83,6 +83,21 @@ void rcu_sync_init(struct rcu_sync *rsp,
>  }
> 
>  /**
> + * rcu_sync_sabotage() - Sabotage a fresh rcu_sync instance
> + * @rsp: Pointer to rcu_sync structure to be sabotaged
> + *
> + * Must be called after rcu_sync_init() and before first use.
> + *
> + * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() pairs
> + * turn into NO-OPs.
> + */
> +void rcu_sync_sabotage(struct rcu_sync *rsp)
> +{
> +	rsp->gp_count++;
> +	rsp->gp_state = !GP_IDLE;

???  A very strange way to say GP_PENDING.  A new GP_DISABLED, perhaps?

							Thanx, Paul

> +}
> +
> +/**
>   * rcu_sync_enter() - Force readers onto slowpath
>   * @rsp: Pointer to rcu_sync structure to use for synchronization
>   *
>