Messages in this thread | | | Date | Tue, 17 May 2011 01:52:41 +0200 | From | Frederic Weisbecker <> | Subject | Re: [GIT PULL rcu/next] rcu commits for 2.6.40 |
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On Mon, May 16, 2011 at 02:24:49PM -0700, Paul E. McKenney wrote: > On Mon, May 16, 2011 at 02:23:29PM +0200, Ingo Molnar wrote: > > > > * Ingo Molnar <mingo@elte.hu> wrote: > > > > > > In the meantime, would you be willing to try out the patch at > > > > https://lkml.org/lkml/2011/5/14/89? This patch helped out Yinghai in > > > > several configurations. > > > > > > Wasn't this the one i tested - or is it a new iteration? > > > > > > I'll try it in any case. > > > > oh, this was a new iteration, mea culpa! > > > > And yes, it solves all problems for me as well. Mind pushing it as a fix? :-) > > ;-) > > Unfortunately, the only reason I can see that it works is (1) there > is some obscure bug in my code or (2) someone somewhere is failing to > call irq_exit() on some interrupt-exit path. Much as I might be tempted > to paper this one over, I believe that we do need to find whatever the > underlying bug is. > > Oh, yes, there is option (3) as well: maybe if an interrupt deschedules > a process, the final irq_exit() is omitted in favor of rcu_enter_nohz()? > But I couldn't see any evidence of this in my admittedly cursory scan > of the x86 interrupt-handling code. > > So until I learn differently, I am assuming that each and every > irq_enter() has a matching call to irq_exit(), and that rcu_enter_nohz() > is called after the final irq_exit() of a given burst of interrupts. > > If my assumptions are mistaken, please do let me know!
About 2), I believe that such an unpairing would have been detected before your whole patchset was merged. For example if an interrupt failed to call rcu_irq_exit(), we would have found cases where we have:
rcu_enter_nohz() <irq> rcu_irq_enter() </irq> rcu_exit_nohz()
And then that last call would trigger "WARN_ON_ONCE(!(rdtp->dynticks & 0x1))".
But may be there was a patch in your set that touched one of these rcu_irq_... callsites.
About 3), it shouldn't happen because preempt_schedule_irq() is called in the exit path of the low level interrupt handler. rcu_exit_irq() is called from the higher level, before resuming to the low level.
That said there might be something nasty that the old checks in the QS APIs were missing.
I think it would be nice to add some checks in rcu-lockdep inside rcu_read_lock()/rcu_dereference() to ensure rdp->dynticks is not even, ie that we are not in an extended qs. That's something I planned to add for my next nohz tasks patchset version, because I bring more dance with the extended quiescent state, but given the problems we are facing today, it may be better sooner.
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