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Date: Sat, 22 Sep 2012 10:26:09 +0200 From: Sasha Levin <levinsasha928@...il.com> To: paulmck@...ux.vnet.ibm.com CC: Michael Wang <wangyun@...ux.vnet.ibm.com>, Dave Jones <davej@...hat.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: RCU idle CPU detection is broken in linux-next On 09/21/2012 05:18 PM, Sasha Levin wrote: > On 09/21/2012 05:12 PM, Paul E. McKenney wrote: >> On Fri, Sep 21, 2012 at 03:26:27PM +0200, Sasha Levin wrote: >>> On 09/21/2012 02:13 PM, Paul E. McKenney wrote: >>>>> This might be unrelated, but I got the following dump as well when trinity >>>>>> decided it's time to reboot my guest: >>>> OK, sounds like we should hold off until you reproduce, then. >>> >>> I'm not sure what you mean. >>> >>> There are basically two issues I'm seeing now, which reproduce pretty much every >>> time: >>> >>> 1. The "using when idle" warning. >>> 2. The rcu related hangs during shutdown. >>> >>> The first one appears early on when I start fuzzing, the other one happens when >>> shutting down - so both of them are reproducible in the same session. >> >> Ah, I misunderstood the "reboot my guest" -- I thought that you were >> doing something like repeated modprobe/rmmod cycles on rcutorture while >> running the guest for an extended time period. That will teach me not >> to reply to email so soon after waking up. ;-) >> >> That said, #2 is expected behavior given the RCU CPU stall warnings in >> your Sept. 20 dmesg. This is because rcutorture does rcu_barrier() on >> the way out, which cannot complete if grace periods are not completing. >> And the later soft lockup is also likely a consequence of the stall, >> because CPU hotplug does a synchronize_sched() while holding the hotplug >> lock, which will then cause get_online_cpus() to hang. >> >> Looking further down, there are hung tasks that are waiting for a >> timeout, but this is also a consequence of the hang because they >> are waiting for MAX_SCHEDULE_TIMEOUT -- in other words, they are >> waiting to be killed at shutdown time. I could suppress this by using >> schedule_timeout_interruptible() in a loop in order to reduce the noise >> in this case. >> >> The remaining traces in that email are also consequences of the stall. >> >> So why the stall? >> >> Using RCU from a CPU that RCU believes to be idle can cause arbitrary >> bad behavior (possibly including stalls), but with very low probability. >> The reason that things can go arbitrarily bad is that RCU is ignoring >> the CPU, and thus not waiting for any RCU read-side critical sections. >> This could of course result in abitrary corruption of memory. The reason >> for the low probability is that grace periods tend to be long and RCU >> read-side critical sections tend to be short. >> >> It looks like you are running -next, which has RCU grace periods driven >> by a kthread. Is it possible that this kthread is not getting a chance >> to run (in fact, the "Stall ended before state dump start" is consistent >> with that possibility), but in that case I would expect to see a soft >> lockup from it. Furthermore, in that case, it would be expected to >> start running again as soon as things started going idle during shutdown. >> >> Or did the system somehow manage to stay busy despite being in shutdown? >> Or, for that matter, are you overcommitting the physical CPUs on your >> trinity test setup? > > Nope, I originally had 4 vcpus in the guest with the host running 4 physical > cpus, but I've also tested it with just 2 vcpus and still see the warnings. Some more info that might help, I'm also occasionally seeing: [ 42.389345] ------------[ cut here ]------------ [ 42.389348] WARNING: at kernel/rcutree.c:375 rcu_eqs_enter+0x5c/0xc0() [ 42.389350] Pid: 0, comm: swapper/2 Tainted: G W 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 [ 42.389351] Call Trace: [ 42.389354] [<ffffffff811c653c>] ? rcu_eqs_enter+0x5c/0xc0 [ 42.389356] [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0 [ 42.389359] [<ffffffff81106975>] warn_slowpath_null+0x15/0x20 [ 42.389361] [<ffffffff811c653c>] rcu_eqs_enter+0x5c/0xc0 [ 42.389364] [<ffffffff811c66f3>] rcu_idle_enter+0x43/0xa0 [ 42.389366] [<ffffffff81078956>] cpu_idle+0x126/0x160 [ 42.389369] [<ffffffff83985a64>] start_secondary+0x26e/0x276 [ 42.389370] ---[ end trace 04c11301284d64ee ]--- [ 42.389394] ------------[ cut here ]------------ [ 42.389396] WARNING: at kernel/rcutree.c:350 rcu_eqs_enter_common+0x709/0x970() [ 42.389398] Pid: 0, comm: swapper/2 Tainted: G W 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 [ 42.389399] Call Trace: [ 42.389402] [<ffffffff811c6019>] ? rcu_eqs_enter_common+0x709/0x970 [ 42.389405] [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0 [ 42.389407] [<ffffffff81106975>] warn_slowpath_null+0x15/0x20 [ 42.389410] [<ffffffff811c6019>] rcu_eqs_enter_common+0x709/0x970 [ 42.389412] [<ffffffff811c658f>] rcu_eqs_enter+0xaf/0xc0 [ 42.389414] [<ffffffff811c66f3>] rcu_idle_enter+0x43/0xa0 [ 42.389417] [<ffffffff81078956>] cpu_idle+0x126/0x160 [ 42.389420] [<ffffffff83985a64>] start_secondary+0x26e/0x276 [ 42.389421] ---[ end trace 04c11301284d64ef ]--- [ 42.389424] ------------[ cut here ]------------ [ 42.389426] WARNING: at kernel/rcutree.c:527 rcu_eqs_exit+0x4f/0xb0() [ 42.389427] Pid: 0, comm: swapper/2 Tainted: G W 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 [ 42.389428] Call Trace: [ 42.389431] [<ffffffff811c6d9f>] ? rcu_eqs_exit+0x4f/0xb0 [ 42.389433] [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0 [ 42.389436] [<ffffffff81106975>] warn_slowpath_null+0x15/0x20 [ 42.389438] [<ffffffff811c6d9f>] rcu_eqs_exit+0x4f/0xb0 [ 42.389441] [<ffffffff811c6f13>] rcu_idle_exit+0x43/0xa0 [ 42.389443] [<ffffffff8107896d>] cpu_idle+0x13d/0x160 [ 42.389445] [<ffffffff83985a64>] start_secondary+0x26e/0x276 [ 42.389447] ---[ end trace 04c11301284d64f0 ]--- [ 42.389448] ------------[ cut here ]------------ [ 42.389450] WARNING: at kernel/rcutree.c:501 rcu_eqs_exit_common+0x4a/0x3a0() [ 42.389451] Pid: 0, comm: swapper/2 Tainted: G W 3.6.0-rc6-next-20120921-sasha-00002-ge9c9495-dirty #378 [ 42.389452] Call Trace: [ 42.389455] [<ffffffff811c679a>] ? rcu_eqs_exit_common+0x4a/0x3a0 [ 42.389458] [<ffffffff81106886>] warn_slowpath_common+0x86/0xb0 [ 42.389460] [<ffffffff81106975>] warn_slowpath_null+0x15/0x20 [ 42.389462] [<ffffffff811c679a>] rcu_eqs_exit_common+0x4a/0x3a0 [ 42.389465] [<ffffffff811c6dec>] rcu_eqs_exit+0x9c/0xb0 [ 42.389467] [<ffffffff811c6f13>] rcu_idle_exit+0x43/0xa0 [ 42.389470] [<ffffffff8107896d>] cpu_idle+0x13d/0x160 [ 42.389472] [<ffffffff83985a64>] start_secondary+0x26e/0x276 [ 42.389474] ---[ end trace 04c11301284d64f1 ]--- Thanks, Sasha -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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