linux-kernel - Re: [PATCH] nohz: Remove tick_nohz_idle_enter_norcu() / tick_nohz_idle_exit

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Message-ID: <CAFTL4hx9m546KRPZX-dH+_ETyLY+WAR5cpOKoCoyECnSX2iHkA@mail.gmail.com>
Date:	Mon, 21 Nov 2011 02:46:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Josh Triplett <josh@...htriplett.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] nohz: Remove tick_nohz_idle_enter_norcu() / tick_nohz_idle_exit_norcu()

2011/11/19 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> On Thu, Nov 17, 2011 at 05:03:44PM -0800, Paul E. McKenney wrote:
>> On Thu, Nov 17, 2011 at 12:11:34PM -0800, Josh Triplett wrote:
>> > On Thu, Nov 17, 2011 at 06:48:14PM +0100, Frederic Weisbecker wrote:
>> > > Those two APIs were provided to optimize the calls of
>> > > tick_nohz_idle_enter() and rcu_idle_enter() into a single
>> > > irq disabled section. This way no interrupt happening in-between would
>> > > needlessly process any RCU job.
>> > >
>> > > Now we are talking about an optimization for which benefits
>> > > have yet to be measured. Let's start simple and completely decouple
>> > > idle rcu and dyntick idle logics to simplify.
>> > >
>> > > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
>> > > Cc: Ingo Molnar <mingo@...hat.com>
>> > > Cc: Thomas Gleixner <tglx@...utronix.de>
>> > > Cc: Peter Zijlstra <peterz@...radead.org>
>> > > Cc: Josh Triplett <josh@...htriplett.org>
>> >
>> > Reviewed-by: Josh Triplett <josh@...htriplett.org>
>>
>> Merged, thank you both!
>
> And here is a patch on top of yours to allow nesting of rcu_idle_enter()
> and rcu_idle_exit().  Thoughts?
>
>                                                        Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Allow nesting of rcu_idle_enter() and rcu_idle_exit()
>
> Running user tasks in dyntick-idle mode requires RCU to undergo
> an idle-to-non-idle transition on each entry into the kernel, and
> vice versa on each exit from the kernel.  However, situations where
> user tasks cannot run in dyntick-idle mode (for example, when there
> is more than one runnable task on the CPU in question) also require
> RCU to undergo an idle-to-non-idle transition when coming out of the
> idle loop (and vice versa when entering the idle loop).

Not sure what you mean about the idle loop with the dyntick-idle mode we
can't enter when we resume to userspace with more than one task in the runqueue.

>  In this case,
> RCU would see one idle-to-non-idle transition when the task became
> runnable, and another when the task executed a system call.

I'm a bit confused with this changelog.

What can happen with the adaptive tickless thing is:

- When we resume to userspace after a syscall/irq/exception and we are
not in RCU extended quiescent state, then switch to it. We may call it RCU
idle mode I guess but that may start to be confusing.
So this may involve several kind of nesting. From a single rcu_idle_enter()
to more complicated scenario if we switch to RCU extended qs from an
an interrupt: rcu_idle_exit() is called on entry of the irq, rcu_idle_enter() is
called in the middle then finally a last call to rcu_idle_enter() in the irq
exit at which point only we want the RCU extended qs to be effective.

- We may also exit that RCU extended qs state by involving other funny
nesting. We have the simple syscall enter that just calls rcu_idle_exit() if
we were in userspace in RCU extended qs. We may also receive an IPI
that enqueues a new task, in which case we may exit the RCU extended
quiescent from the irq with the following nesting:
rcu_idle_exit() on irq entry, then another call to rcu_idle_exit() to prevent
from resuming the RCU extended quiescent state when we come back
to userspace and finally the rcu_idle_enter() in the irq exit.

Is that what you had in mind?

Thanks.
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