linux-kernel - Re: [PATCH RFC] rcu: Make __rcu_read

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Date:	Mon, 26 Mar 2012 22:15:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC] rcu: Make __rcu_read_lock() inlinable

On Mon, Mar 26, 2012 at 08:47:10PM +0200, Peter Zijlstra wrote:
> On Mon, 2012-03-26 at 11:32 -0700, Paul E. McKenney wrote:
> > 
> > I could inline them into sched.h, if you are agreeable.
> 
> Sure, or put it in kernel/sched/core.c.

That was my first thought, but there is a use of switch_to() in
arch/um/drivers/mconsole_kern.c.

> > I am a bit concerned about putting them both together because I am betting
> > that at least some of the architectures having tracing in switch_to(),
> > which I currently do not handle well. 
> 
> I would hope not.. there's a generic trace_sched_switch() and
> switch_to() is supposed to be black magic. I'd be fine breaking that as
> long as we can detect it.

Hmmm...  I am not yet sure whether it is easier to make RCU use legal
in switch_to() or to detect it.  I am inclined to take whatever course
is easiest, which is likely to make it legal.  :-/

> >  At the moment, the ways I can
> > think of to handle it well require saving before the switch and restoring
> > afterwards.  Otherwise, I can end up with the ->rcu_read_unlock_special
> > flags getting associated with the wrong RCU read-side critical section,
> > as happened last year.
> > 
> > Preemption is disabled at this point, correct?
> 
> Yeah, and soon we'll have interrupts disabled as well (on all archs,
> currently only ARM has interrupts enabled at this point).

Good to know!

> > Hmmm...  One way that I could reduce the overhead that preemptible RCU
> > imposes on the scheduler would be to move the task_struct queuing from
> > its current point upon entry to the scheduler to just before switch_to().
> > (The _bh and _sched quiescent states still need to remain at scheduler
> > entry.)  That would mean that RCU would not queue tasks that entered
> > the scheduler, but did not actually do a context switch.
> 
> That would make sense anyhow, right? No point in queueing a task if you
> didn't actually switch away from it.

Also it would simplify the save and restore operation, I believe.

> > Would that be helpful?
> 
> For now that's preemptible rcu only, and as such a somewhat niche
> feature (iirc its not enabled in the big distros) so I don't think it
> matters too much. But yeah, would be nice.

OK, let me see what works best.

							Thanx, Paul

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