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

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Message-ID: <20140805205711.7a52076c@gandalf.local.home>
Date:	Tue, 5 Aug 2014 20:57:11 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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, peterz@...radead.org,
	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 Sat, 2 Aug 2014 15:58:57 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Sat, Aug 02, 2014 at 04:47:19PM +0200, Oleg Nesterov wrote:
> > On 08/01, Paul E. McKenney wrote:
> > >
> > > On Fri, Aug 01, 2014 at 11:32:51AM -0700, Paul E. McKenney wrote:
> > > > On Fri, Aug 01, 2014 at 05:09:26PM +0200, Oleg Nesterov wrote:
> > > > > On 07/31, Paul E. McKenney wrote:
> > > > > >
> > > > > > +void synchronize_rcu_tasks(void)
> > > > > > +{
> > > > > > +	/* Complain if the scheduler has not started.  */
> > > > > > +	rcu_lockdep_assert(!rcu_scheduler_active,
> > > > > > +			   "synchronize_rcu_tasks called too soon");
> > > > > > +
> > > > > > +	/* Wait for the grace period. */
> > > > > > +	wait_rcu_gp(call_rcu_tasks);
> > > > > > +}
> > > > >
> > > > > Btw, what about CONFIG_PREEMPT=n ?
> > > > >
> > > > > I mean, can't synchronize_rcu_tasks() be synchronize_sched() in this
> > > > > case?
> > > >
> > > > Excellent point, indeed it can!
> > > >
> > > > And if I do it right, it will make CONFIG_TASKS_RCU=y safe for kernel
> > > > tinification.  ;-)
> > >
> > > Unless, that is, we need to wait for trampolines in the idle loop...
> > >
> > > Sounds like a question for Steven.  ;-)
> > 
> > Sure, but the full blown synchronize_rcu_tasks() can't handle the idle threads
> > anyway. An idle thread can not be deactivated and for_each_process() can't see
> > it anyway.
> 
> Indeed, if idle threads need to be tracked, their tracking will need to
> be at least partially special-cased.
> 

Yeah, idle threads can be affected by the trampolines. That is, we can
still hook a trampoline to some function in the idle loop.

But we should be able to make the hardware call that puts the CPU to
sleep a quiescent state too. May need to be arch dependent. :-/

-- Steve
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