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

Frederic Weisbecker
2011-09-30T13:11:16Z

On Thu, Sep 29, 2011 at 10:12:05AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 29, 2011 at 02:30:44PM +0200, Frederic Weisbecker wrote:
> > 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.

Should I wait for your updated patch before rebasing?

>
> > > > > 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.

Right.

>
> > > 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.

Yep.

> > > 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.

Fine. Just need to know if they need an update against a patch from you
that is to come or something.