linux-kernel - Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu

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Date:	Tue, 29 Jul 2014 11:06:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Tue, Jul 29, 2014 at 07:33:32PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 29, 2014 at 10:23:04AM -0700, Paul E. McKenney wrote:
> > On Tue, Jul 29, 2014 at 10:14:16AM +0200, Peter Zijlstra wrote:
> > > On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> > > > @@ -254,6 +254,8 @@ void rcu_check_callbacks(int cpu, int user)
> > > >  		rcu_sched_qs(cpu);
> > > >  	else if (!in_softirq())
> > > >  		rcu_bh_qs(cpu);
> > > > +	if (user)
> > > > +		rcu_note_voluntary_context_switch(current);
> > > >  }
> > > 
> > > There's nothing like sending email you can't find something... :-)
> > 
> > Well, this is unfortunately only a partial solution.  It does not handle
> > the NO_HZ_FULL scheduling-clock-free usermode execution.  I have ink on
> > paper indicating a couple of ways to do that, but figured I should get
> > feedback on this stuff before going too much farther.
> 
> Yah, so the nohz_full already has the horrid overhead of user<->kernel
> switches, so you can 'trivially' hook into those.

Yep, the plan is to use RCU's dyntick-idle code as the hook.

> FWIW its _the_ thing that makes nohz_full uninteresting for me. The
> required overhead is insane. But yes there are people willing to pay
> that etc..

It would indeed be good to reduce the overhead.  I could imagine all sorts
of insane approaches involving assuming that CPU write buffers flush in
bounded time, though CPU vendors seem unwilling to make guarantees in
this area.  ;-)

Or is something other than rcu_user_enter() and rcu_user_exit() causing
the pain here?

							Thanx, Paul

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