linux-kernel - Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle tasks

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Date:	Wed, 13 Aug 2014 09:51:32 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle
 tasks

On Wed, 13 Aug 2014 15:40:25 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Aug 13, 2014 at 05:48:18AM -0700, Paul E. McKenney wrote:
> > On Wed, Aug 13, 2014 at 10:12:15AM +0200, Peter Zijlstra wrote:
> > > On Mon, Aug 11, 2014 at 03:49:04PM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > > 
> > > > Because idle-task code may need to be patched, RCU-tasks need to wait
> > > > for idle tasks to schedule.  This commit therefore detects this case
> > > > via context switch.  Block CPU hotplug during this time to avoid sending
> > > > IPIs to offline CPUs.
> > > > 
> > > > Note that checking for changes in the dyntick-idle counters is tempting,
> > > > but wrong.  The reason that it is wrong is that a interrupt or NMI can
> > > > increment these counters without necessarily allowing the idle tasks to
> > > > make any forward progress.
> > > 
> > > I'm going to NAK this.. with that rcu_idle patch I send there's
> > > typically only a single idle function thats out of bounds and if its
> > > more it can be made that with a bit of tlc to the cpuidle driver in
> > > question.
> > > 
> > > This needs _FAR_ more justification than a maybe and a want.
> > 
> > Peter, your patch might be a good start, but I didn't see any reaction
> > from Steven or Masami and it did only x86.
> 
> That's not an excuse for doing horrible things. And inventing new infra
> that needs to wake all CPUs is horrible.

I still need to look at the patches, but if this is just for the idle
case, then we don't need it. The idle case can be solved with a simple
sched_on_each_cpu(). I need a way to solve waiting for processes to
finish from a preemption point.

That's all I want, and if we can remove the "idle" case and document it
well that it's not covered and a sched_on_each_cpu() may be needed,
then I'm fine with that.

	sched_on_each_cpu(dummy_op);
	call_rcu_tasks(free_tramp);

Would that work?

-- Steve
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