On Wed, Jul 26, 2017 at 10:50:13AM -0700, Paul E. McKenney wrote: > On Wed, Jul 26, 2017 at 09:54:32AM -0700, David Miller wrote: > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > > Date: Wed, 26 Jul 2017 08:49:00 -0700 > > > > > On Wed, Jul 26, 2017 at 04:33:40PM +0100, Jonathan Cameron wrote: > > >> Didn't leave it long enough. Still bad on 4.10-rc7 just took over > > >> an hour to occur. > > > > > > And it is quite possible that SOFTLOCKUP_DETECTOR=y and HZ_PERIODIC=y > > > are just greatly reducing the probability of the problem rather than > > > completely preventing it. > > > > > > Still, hopefully useful information, thank you for the testing! > > > > I guess that invalidates my idea to test reverting recent changes to > > the tick-sched.c code... :-/ > > > > In NO_HZ_IDLE mode, what is really supposed to happen on a completely > > idle system? > > > > All the cpus enter the idle loop, have no timers programmed, and they > > all just go to sleep until an external event happens. > > > > What ensures that grace periods get processed in this regime? > > There are several different situations with different mechanisms: > > 1. No grace period is in progress and no RCU callbacks are pending > anywhere in the system. In this case, some other event would > need to start a grace period, so RCU just stays idle until that > happens, possibly indefinitely. According to the battery-powered > embedded guys, this is a feature, not a bug. ;-) > > 2. No grace period is in progress, but there is at least one RCU > callback somewhere in the system. In this case, the mechanism > depends on CONFIG_RCU_FAST_NO_HZ: > > CONFIG_RCU_FAST_NO_HZ=n: The CPU on which the callback is > queued will return "true" in response to the call to > rcu_needs_cpu() that is made shortly before that CPU > enters idle. This will cause the scheduling-clock > interrupt to remain on, despite the CPU being idle, > which will in turn allow RCU's state machine to continue > running out of softirq, triggered by the scheduling-clock > interrupts. > > CONFIG_RCU_FAST_NO_HZ=y: The CPU on which the callback is queued > will return "false" in response to the call to > rcu_needs_cpu() that is made shortly before that CPU > enters idle. However, it will also request a next event > about six seconds in the future if all callbacks do > nothing but free memory (kfree_rcu()), or about four > jiffies in the future if at least one callback does > something more than just free memory. > > There is also a rcu_prepare_for_idle() function that > is invoked later in the idle-entry process in this case > which will wake up the grace-period kthread if need be. > > 3. A grace period is in progress. In this case the grace-period > kthread is either currently running (in which case there will be > at least one non-idle CPU) or is in a timed wait for its next > scan for idle/offline CPUs (such CPUs need the grace-period > kthread to report quiescent states on their behalf). In this > latter case, the timer subsystem will post a next event that > will be the wakeup time for the grace-period kthread, or some > earlier event. > > This is where we have been seeing trouble, if for no other > reason because RCU CPU stall warnings only happen when there > is a grace period in progress. > > That is the theory, anyway... > > And when I enabled CONFIG_SOFTLOCKUP_DETECTOR, I still see failures. > I did 24 half-hour rcutorture runs on the TREE01 scenario, and two of them > saw RCU CPU stall warnings with starvation of the grace-period kthread. > I just now started another test but without CONFIG_SOFTLOCKUP_DETECTOR > to see if it makes a significance difference for my testing. I do have > CONFIG_RCU_FAST_NO_HZ=y in my runs. And without CONFIG_SOFTLOCKUP_DETECTOR, I see five runs of 24 with RCU CPU stall warnings. So it seems likely that CONFIG_SOFTLOCKUP_DETECTOR really is having an effect. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe sparclinux" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
