Messages in this thread | | | Date | Thu, 29 Sep 2011 10:12:05 -0700 | From | "Paul E. McKenney" <> | Subject | Re: linux-next-20110923: warning kernel/rcutree.c:1833 |
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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
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