Re: linux-next-20110923: warning kernel/rcutree.c:1833

Paul E. McKenney
2011-09-29T17:14:09Z

On Thu, Sep 29, 2011 at 02:30:44PM +0200, Frederic Weisbecker wrote:
> On Wed, Sep 28, 2011 at 05:55:45PM -0700, Paul E. McKenney wrote:
> > On Thu, Sep 29, 2011 at 01:46:36AM +0200, Frederic Weisbecker wrote:
> > > Not sure what you mean. You want to split that specific patch or
> > > others?
> >
> > It looks to me that having my pair of patches on top of yours is
> > really ugly. If we are going to introduce the per-CPU idle variable,
> > we should make a patch stack that uses that from the start. This allows
> > me to bisect to track down the failures I am seeing on Power.
>
> Yeah right. My patches fix the use on extended qs in idle. But if
> idle itself is considered as a quiescent state all along, that's about
> useless. So it sounds indeed better in that order.
>
> > If you are too busy, I can take this on, but we might get better results
> > if you did it. (And I certainly cannot complain about the large amount
> > of time and energy that you have put into this -- plus the reduction in
> > OS jitter will be really cool to have!)
>
> No problem, I can take it.

Very good! Rebasing and testing going well thus far.

> > > > > Although idle and rcu/nohz are still close notions, it sounds
> > > > > more logical the other way around in the ordering:
> > > > >
> > > > > tick_nohz_idle_enter() {
> > > > > rcu_idle_enter() {
> > > > > rcu_enter_nohz();
> > > > > }
> > > > > }
> > > > >
> > > > > tick_nohz_irq_exit() {
> > > > > rcu_idle_enter() {
> > > > > rcu_enter_nohz();
> > > > > }
> > > > > }
> > > > >
> > > > > Because rcu ext qs is something used by idle, not the opposite.
> >
> > Re-reading this makes me realize that I would instead say that idle
> > is an example of an RCU extended quiescent state, or that the rcu_ext_qs
> > argument to the various functions is used to indicate whether or not
> > we are immediately entering/leaving idle from RCU's viewpoint.
> >
> > So what were you really trying to say here? ;-)
>
> I was thinking about the fact that idle is a caller of rcu_enter_nohz().
> And there may be more callers of it in the future. So I thought it may
> be better to keep rcu_enter_nohz() idle-agnostic.
>
> But it's fine, there are other ways to call rcu_idle_enter()/rcu_idle_exit()
> from the right places other than from rcu_enter/exit_nohz().
> We have tick_check_idle() on irq entry and tick_nohz_irq_exit(), both are called
> on the first interrupt level in idle.
>
> So I can change that easily for the nohz cpusets.

Heh! From what I can see, we were both wrong!

My thought at this point is to make it so that rcu_enter_nohz() and
rcu_exit_nohz() are renamed to rcu_enter_idle() and rcu_exit_idle()
respectively. I drop the per-CPU variable and the added functions
from one of my patches. These functions, along with rcu_irq_enter(),
rcu_irq_exit(), rcu_nmi_enter(), and rcu_nmi_exit(), are moved out from
under CONFIG_NO_HZ. This allows these functions to track idle state
regardless of the setting of CONFIG_NO_HZ. It also separates the state
of the scheduling-clock tick from RCU's view of CPU idleness, which
simplifies things.

I will put something together along these lines.

> > > > The problem I have with this is that it is rcu_enter_nohz() that tracks
> > > > the irq nesting required to correctly decide whether or not we are going
> > > > to really go to idle state. Furthermore, there are cases where we
> > > > do enter idle but do not enter nohz, and that has to be handled correctly
> > > > as well.
> > > >
> > > > Now, it is quite possible that I am suffering a senior moment and just
> > > > failing to see how to structure this in the design where rcu_idle_enter()
> > > > invokes rcu_enter_nohz(), but regardless, I am failing to see how to
> > > > structure this so that it works correctly.
> > > >
> > > > Please feel free to enlighten me!
> > >
> > > Ah I realize that you want to call rcu_idle_exit() when we enter
> > > the first level interrupt and rcu_idle_enter() when we exit it
> > > to return to idle loop.
> > >
> > > But we use that check:
> > >
> > > if (user ||
> > > (rcu_is_cpu_idle() &&
> > > !in_softirq() &&
> > > hardirq_count() <= (1 << HARDIRQ_SHIFT)))
> > > rcu_sched_qs(cpu);
> > >
> > > So we ensure that by the time we call rcu_check_callbacks(), we are not nesting
> > > in another interrupt.
> >
> > But I would like to enable checks for entering/exiting idle while
> > within an RCU read-side critical section. The idea is to move
> > the checks from their currently somewhat problematic location in
> > rcu_needs_cpu_quick_check() to somewhere more sensible. My current
> > thought is to move them rcu_enter_nohz() and rcu_exit_nohz() near the
> > calls to rcu_idle_enter() and rcu_idle_exit(), respectively.
>
> So, checking if we are calling rcu_idle_enter() while in an RCU
> read side critical section?
>
> But we already have checks that RCU read side API are not called in
> extended quiescent state.

Both checks are good. The existing checks catch this kind of error:

1. CPU 0 goes idle, entering an RCU extended quiescent state.
2. CPU 0 illegally enters an RCU read-side critical section.

The new check catches this kind of error:

1. CPU 0 enters an RCU read-side critical section.
2. CPU 0 goes idle, entering an RCU extended quiescent state,
but illegally so because it is still in an RCU read-side
critical section.

> > This would mean that they operated only in NO_HZ kernels with lockdep
> > enabled, but I am good with that because to do otherwise would require
> > adding nesting-level counters to the non-NO_HZ case, which I would like
> > to avoid, expecially for TINY_RCU.

And my reworking of RCU's NO_HZ code to instead be idle code removes
the NO_HZ-only restriction. Getting rid of the additional per-CPU
variable reduces the TINY_RCU overhead to acceptable levels.

> There can be a secondary check in rcu_read_lock_held() and friends to
> ensures that rcu_is_idle_cpu(). In the non-NO_HZ case it's useful to
> find similar issues.
>
> In fact we could remove the check for rcu_extended_qs() in read side
> APIs and check instead rcu_is_idle_cpu(). That would work in any
> config and not only NO_HZ.
>
> But I hope we can actually keep the check for RCU extended quiescent
> state so that when rcu_enter_nohz() is called from other places than
> idle, we are ready for it.
>
> I believe it's fine to have both checks in PROVE_RCU.

Agreed, I have not yet revisited rcu_extended_qs(), but some change
might be useful.

> > > That said we found RCU uses after we decrement the hardirq offset and until
> > > we reach rcu_irq_exit(). So rcu_check_callbacks() may miss these places
> > > and account spurious quiescent states.
> > >
> > > But between sub_preempt_count() and rcu_irq_exit(), irqs are disabled
> > > AFAIK so we can't be interrupted by rcu_check_callbacks(), except during the
> > > softirqs processing. But we have that ordering:
> > >
> > > add_preempt_count(SOTFIRQ_OFFSET)
> > > local_irq_enable()
> > >
> > > do softirqs
> > >
> > > local_irq_disable()
> > > sub_preempt_count(SOTFIRQ_OFFSET)
> > >
> > > So the !in_softirq() check covers us during the time we process softirqs.
> > >
> > > The only assumption we need is that there is no place between
> > > sub_preempt_count(IRQ_EXIT_OFFSET) and rcu_irq_ext() that has
> > > irqs enabled and that is an rcu read side critical section.
> > >
> > > I'm not aware of any automatic check to ensure that though.
> >
> > Nor am I, which is why I am looking to the checks in
> > rcu_enter_nohz() and rcu_exit_nohz() called out above.
>
> Yep.
>
> > > Anyway, the delta patch looks good.
> >
> > OK, my current plans are to start forward-porting to -rc8, and I would
> > like to have this pair of delta patches or something like them pulled
> > into your stack.
>
> Sure I can take your patches (I'm going to merge the delta into the first).
> But if you want a rebase against -rc8, it's going to be easier if you
> do that rebase on the branch you want me to work on. Then I work on top
> of it.
>
> For example we can take your rcu/dynticks, rewind to
> "rcu: Make synchronize_sched_expedited() better at work sharing"
> 771c326f20029a9f30b9a58237c9a5d5ddc1763d, rebase on top of -rc8
> and I rebase my patches (yours included) on top of it and I repost.
>
> Right?

Yep! Your earlier three patches look to need some extended-quiescent-state
rework as well:

b5566f3d: Detect illegal rcu dereference in extended quiescent state
ee05e5a4: Inform the user about dynticks-idle mode on PROVE_RCU warning
fa5d22cf: Warn when rcu_read_lock() is used in extended quiescent state

So I will leave these out and let you rebase them.

> > > Just a little thing:
> > >
> > > > -void tick_nohz_idle_exit(void)
> > > > +void tick_nohz_idle_exit(bool rcu_ext_qs)
> > >
> > > It becomes weird to have both idle_enter/idle_exit having
> > > that parameter.
> > >
> > > Would it make sense to have tick_nohz_idle_[exit|enter]_norcu()
> > > and a version without norcu?
> >
> > Given that we need to make this work in CONFIG_NO_HZ=n kernels, I believe
> > that the current API is OK. But if you would like to change the API
> > during the forward-port to -rc8, I am also OK with the alternative API
> > you suggest.
>
> Fine. I'll do that rename.

Works for me! ;-)

Thanx, Paul