On Fri, Jun 03, 2011 at 10:56:20PM +0300, Sasha Levin wrote: > On Fri, 2011-06-03 at 12:31 -0700, Paul E. McKenney wrote: > > On Fri, Jun 03, 2011 at 10:54:19AM +0300, Sasha Levin wrote: > > > On Fri, 2011-06-03 at 09:34 +0200, Ingo Molnar wrote: > > > > * Sasha Levin <levinsasha928@xxxxxxxxx> wrote: > > > > > > > > > > with no apparent progress being made. > > > > > > > > > > Since it's something that worked in 2.6.37, I've looked into it to > > > > > find what might have caused this issue. > > > > > > > > > > I've bisected guest kernels and found that the problem starts with: > > > > > > > > > > a26ac2455ffcf3be5c6ef92bc6df7182700f2114 is the first bad commit > > > > > commit a26ac2455ffcf3be5c6ef92bc6df7182700f2114 > > > > > Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx> > > > > > Date: Wed Jan 12 14:10:23 2011 -0800 > > > > > > > > > > rcu: move TREE_RCU from softirq to kthread > > > > > > > > > > Ingo, could you confirm that the problem goes away for you when you > > > > > use an earlier commit? > > > > > > > > testing will have to wait, but there's a recent upstream fix: > > > > > > > > d72bce0e67e8: rcu: Cure load woes > > > > > > > > That *might* perhaps address this problem too. > > > > > > > I've re-tested with Linus's current git, the problem is still there. > > > > > > > If not then this appears to be some sort of RCU related livelock with > > > > brutally overcommitted vcpus. On native this would show up too, in a > > > > less drastic form, as a spurious bootup delay. > > > > > > I don't think it was overcommited by *that* much. With that commit it > > > usually hangs at 20-40 vcpus, while without it I can go up to 255. > > > > Here is a diagnostic patch, untested. It assumes that your system > > has only a few CPUs (maybe 8-16) and that timers are still running. > > It dumps out some RCU state if grace periods extend for more than > > a few seconds. > > > > To activate it, call rcu_diag_timer_start() from process context. > > To stop it, call rcu_diag_timer_stop(), also from process context. > > Since the hang happens in guest kernel very early during boot, I can't > call rcu_diag_timer_start(). What would be a good place to put the > _start() code instead? Assuming that the failure occurs in the host OS rather than in the guest OS, I suggest placing rcu_diag_timer_start() in the host code that starts up the guest. On the other hand, if the failure is occuring in the guest OS, then I suggest placing the call to rcu_diag_timer_start() just after timer initialization -- that is, assuming that interrupts are enabled at the time of the failure. If interrupts are not yet enabled at the time of the failure, color me clueless. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
