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

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Date:	Fri, 15 Jul 2016 06:39:28 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, tj@...nel.org,
	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 Fri, Jul 15, 2016 at 03:27:09PM +0200, Oleg Nesterov wrote:
> 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.

Hmmm...  Why not just move the checks out into the caller?  That
would make the intent much more clear.

							Thanx, Paul

> 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.
>