linux-kernel - Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks

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Date:	Thu, 7 Aug 2014 17:26:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period
 waiting for RCU-tasks

On Thu, Aug 07, 2014 at 08:00:31AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 07, 2014 at 10:45:44AM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 06, 2014 at 03:45:18PM -0700, Paul E. McKenney wrote:
> > > > > > But I still very much hate the polling stuff...
> > > 
> > > The nice thing about the polling approach is minimal overhead in the
> > > common case where RCU-tasks is not in use.
> > 
> > No, quite the reverse, there is overhead when its not in use, as opposed
> > to no overhead at all.
> 
> Say what???
> 
> > I'm still not convinced we need this 'generic' rcu-task stuff and create
> > yet another kthread with polling semantics, we want to let the system
> > idle out when there's nothing to do, not keep waking it up.
> 
> Which is exactly what happens.  The kthread is created only at first
> use, so if no one uses RCU-tasks, then no kthread is created, see
> https://lkml.org/lkml/2014/8/4/630.  Even if a kthread is created, if
> there is no more work for it to do, it sleeps indefinitely.  See for
> example https://lkml.org/lkml/2014/8/4/629.

Ah, the 'full' patch I was staring at for reference did an unconditional
poll.

> > So do we really need the call_rcu_task() thing and why isn't something
> > like synchronize_tasks() good enough?
> 
> Sounds like a question for Steven.
> 
> > So the thing is, the one proposed user is very rare (*) and for that
> > you're adding overhead outside of that user (a separate kthread) and
> > your adding overhead when its not used.
> 
> If that really was the case, that would be bad.  However, in the latest
> versions, that is no longer the case.
> 
> > * I'm assuming that, since tracing is 'rare' and this is some tracing
> > thing.
> 
> Another good point for Steven.

Yes.. and he's back now, so please :-)

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