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. 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
