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

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Date:	Fri, 15 Jul 2016 15:27:09 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, tj@...nel.org,
	paulmck@...ux.vnet.ibm.com, john.stultz@...aro.org,
	dimitrysh@...gle.com, romlem@...gle.com, ccross@...gle.com,
	tkjos@...gle.com
Subject: Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

On 07/14, 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?

OK, agreed,

> ---
> --- 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);

Heh ;) I too think it should be renamed. I'd suggest

	__rcu_sync_enter(rss);

although I do not really mind and agree with any name.

Hopefully I'll send some cleanups soon, rcu_sync_enter() will be
re-implemented as

	rcu_sync_enter(rss)
	{
		if (__rcu_sync_enter(rss))
			__rcu_sync_wait(rss);
	}

> + * 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.

OK. And after those cleanups __rcu_sync_enter() can be called at any
time.

Oleg.