Re: INFO: task hung in perf_trace_event_unreg

Dmitry Vyukov
2018-04-11T10:06:51Z

On Tue, Apr 10, 2018 at 7:02 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>> >> >> On Mon, Apr 2, 2018 at 7:23 PM, Paul E. McKenney
>> >> >> <paulmck@linux.vnet.ibm.com> wrote:
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> > Hello,
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> > syzbot hit the following crash on upstream commit
>> >> >> >> >> >> >> > 0adb32858b0bddf4ada5f364a84ed60b196dbcda (Sun Apr 1 21:20:27 2018 +0000)
>> >> >> >> >> >> >> > Linux 4.16
>> >> >> >> >> >> >> > syzbot dashboard link:
>> >> >> >> >> >> >> > https://syzkaller.appspot.com/bug?extid=2dbc55da20fa246378fd
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> > Unfortunately, I don't have any reproducer for this crash yet.
>> >> >> >> >> >> >> > Raw console output:
>> >> >> >> >> >> >> > https://syzkaller.appspot.com/x/log.txt?id=5487937873510400
>> >> >> >> >> >> >> > Kernel config:
>> >> >> >> >> >> >> > https://syzkaller.appspot.com/x/.config?id=-2374466361298166459
>> >> >> >> >> >> >> > compiler: gcc (GCC) 7.1.1 20170620
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> >> >> >> >> >> >> > Reported-by: syzbot+2dbc55da20fa246378fd@syzkaller.appspotmail.com
>> >> >> >> >> >> >> > It will help syzbot understand when the bug is fixed. See footer for
>> >> >> >> >> >> >> > details.
>> >> >> >> >> >> >> > If you forward the report, please keep this part and the footer.
>> >> >> >> >> >> >> >
>> >> >> >> >> >> >> > REISERFS warning (device loop4): super-6502 reiserfs_getopt: unknown mount
>> >> >> >> >> >> >> > option "g �;e�K�׫>pquota"
>> >> >> >> >> >> >
>> >> >> >> >> >> > Might not hurt to look into the above, though perhaps this is just syzkaller
>> >> >> >> >> >> > playing around with mount options.
>> >> >> >> >> >> >
>> >> >> >> >> >> >> > INFO: task syz-executor3:10803 blocked for more than 120 seconds.
>> >> >> >> >> >> >> > Not tainted 4.16.0+ #10
>> >> >> >> >> >> >> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> >> >> >> >> >> >> > syz-executor3 D20944 10803 4492 0x80000002
>> >> >> >> >> >> >> > Call Trace:
>> >> >> >> >> >> >> > context_switch kernel/sched/core.c:2862 [inline]
>> >> >> >> >> >> >> > __schedule+0x8fb/0x1ec0 kernel/sched/core.c:3440
>> >> >> >> >> >> >> > schedule+0xf5/0x430 kernel/sched/core.c:3499
>> >> >> >> >> >> >> > schedule_timeout+0x1a3/0x230 kernel/time/timer.c:1777
>> >> >> >> >> >> >> > do_wait_for_common kernel/sched/completion.c:86 [inline]
>> >> >> >> >> >> >> > __wait_for_common kernel/sched/completion.c:107 [inline]
>> >> >> >> >> >> >> > wait_for_common kernel/sched/completion.c:118 [inline]
>> >> >> >> >> >> >> > wait_for_completion+0x415/0x770 kernel/sched/completion.c:139
>> >> >> >> >> >> >> > __wait_rcu_gp+0x221/0x340 kernel/rcu/update.c:414
>> >> >> >> >> >> >> > synchronize_sched.part.64+0xac/0x100 kernel/rcu/tree.c:3212
>> >> >> >> >> >> >> > synchronize_sched+0x76/0xf0 kernel/rcu/tree.c:3213
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> I don't think this is a perf issue. Looks like something is preventing
>> >> >> >> >> >> >> rcu_sched from completing. If there's a CPU that is running in kernel
>> >> >> >> >> >> >> space and never scheduling, that can cause this issue. Or if RCU
>> >> >> >> >> >> >> somehow missed a transition into idle or user space.
>> >> >> >> >> >> >
>> >> >> >> >> >> > The RCU CPU stall warning below strongly supports this position ...
>> >> >> >> >> >>
>> >> >> >> >> >> I think this is this guy then:
>> >> >> >> >> >>
>> >> >> >> >> >> https://syzkaller.appspot.com/bug?id=17f23b094cd80df750e5b0f8982c521ee6bcbf40
>> >> >> >> >> >>
>> >> >> >> >> >> #syz dup: INFO: rcu detected stall in __process_echoes
>> >> >> >> >> >
>> >> >> >> >> > Seems likely to me!
>> >> >> >> >> >
>> >> >> >> >> >> Looking retrospectively at the various hang/stall bugs that we have, I
>> >> >> >> >> >> think we need some kind of priority between them. I.e. we have rcu
>> >> >> >> >> >> stalls, spinlock stalls, workqueue hangs, task hangs, silent machine
>> >> >> >> >> >> hang and maybe something else. It would be useful if they fire
>> >> >> >> >> >> deterministically according to priorities. If there is an rcu stall,
>> >> >> >> >> >> that's always detected as CPU stall. Then if there is no RCU stall,
>> >> >> >> >> >> but a workqueue stall, then that's always detected as workqueue stall,
>> >> >> >> >> >> etc.
>> >> >> >> >> >> Currently if we have an RCU stall (effectively CPU stall), that can be
>> >> >> >> >> >> detected either RCU stall or a task hung, producing 2 different bug
>> >> >> >> >> >> reports (which is bad).
>> >> >> >> >> >> One can say that it's only a matter of tuning timeouts, but at least
>> >> >> >> >> >> task hung detector has a problem that if you set timeout to X, it can
>> >> >> >> >> >> detect hung anywhere between X and 2*X. And on one hand we need quite
>> >> >> >> >> >> large timeout (a minute may not be enough), and on the other hand we
>> >> >> >> >> >> can't wait for an hour just to make sure that the machine is indeed
>> >> >> >> >> >> dead (these things happen every few minutes).
>> >> >> >> >> >
>> >> >> >> >> > I suppose that we could have a global variable that was set to the
>> >> >> >> >> > priority of the complaint in question, which would suppress all
>> >> >> >> >> > lower-priority complaints. Might need to be opt-in, though -- I would
>> >> >> >> >> > guess that not everyone is going to be happy with one complaint suppressing
>> >> >> >> >> > others, especially given the possibility that the two complaints might
>> >> >> >> >> > be about different things.
>> >> >> >> >> >
>> >> >> >> >> > Or did you have something more deft in mind?
>> >> >> >> >>
>> >> >> >> >>
>> >> >> >> >> syzkaller generally looks only at the first report. One does not know
>> >> >> >> >> if/when there will be a second one, or the second one can be induced
>> >> >> >> >> by the first one, and we generally want clean reports on a non-tainted
>> >> >> >> >> kernel. So we don't just need to suppress lower priority ones, we need
>> >> >> >> >> to produce the right report first.
>> >> >> >> >> I am thinking maybe setting:
>> >> >> >> >> - rcu stalls at 1.5 minutes
>> >> >> >> >> - workqueue stalls at 2 minutes
>> >> >> >> >> - task hungs at 2.5 minutes
>> >> >> >> >> - and no output whatsoever at 3 minutes
>> >> >> >> >> Do I miss anything? I think at least spinlocks. Should they go before
>> >> >> >> >> or after rcu?
>> >> >> >> >
>> >> >> >> > That is what I know of, but the Linux kernel being what it is, there is
>> >> >> >> > probably something more out there. If not now, in a few months. The
>> >> >> >> > RCU CPU stall timeout can be set on the kernel-boot command line, but
>> >> >> >> > you probably already knew that.
>> >> >> >>
>> >> >> >> Well, it's all based solely on a large number of patches and stopgaps.
>> >> >> >> If we fix main problems for today, it's already good.
>> >> >> >
>> >> >> > Fair enough!
>> >> >> >
>> >> >> >> > Just for comparison, back in DYNIX/ptx days the RCU CPU stall timeout
>> >> >> >> > was 1.5 -seconds-. ;-)
>> >> >> >>
>> >> >> >> Have you tried to instrument every basic block with a function call to
>> >> >> >> collect coverage, check every damn memory access for validity, enable
>> >> >> >> all thinkable and unthinkable debug configs and put the insanest load
>> >> >> >> one can imagine from a swarm of parallel threads? It makes things a
>> >> >> >> bit slower ;)
>> >> >> >
>> >> >> > Given that we wouldn't have had enough CPU or memory to accommodate
>> >> >> > all of that back in DYNIX/ptx days, I am forced to answer "no". ;-)
>> >> >> >
>> >> >> >> >> This will require fixing task hung. Have not yet looked at workqueue detector.
>> >> >> >> >> Does at least RCU respect the given timeout more or less precisely?
>> >> >> >> >
>> >> >> >> > Assuming that there is at least one CPU capable of taking scheduling-clock
>> >> >> >> > interrupts, it should respect the timeout to within a few jiffies.
>> >> >>
>> >> >>
>> >> >> Hi Paul,
>> >> >>
>> >> >> Speaking of stalls and rcu, we are seeing lots of crashes that go like this:
>> >> >>
>> >> >> INFO: rcu_sched self-detected stall on CPU[ 404.992530] INFO:
>> >> >> rcu_sched detected stalls on CPUs/tasks:
>> >> >> INFO: rcu_sched self-detected stall on CPU[ 454.347448] INFO:
>> >> >> rcu_sched detected stalls on CPUs/tasks:
>> >> >> INFO: rcu_sched self-detected stall on CPU[ 396.073634] INFO:
>> >> >> rcu_sched detected stalls on CPUs/tasks:
>> >> >>
>> >> >> or like this:
>> >> >>
>> >> >> INFO: rcu_sched self-detected stall on CPU
>> >> >> INFO: rcu_sched detected stalls on CPUs/tasks:
>> >> >> 0-....: (125000 ticks this GP) idle=0ba/1/4611686018427387906
>> >> >> softirq=57641/57641 fqs=31151
>> >> >> 0-....: (125000 ticks this GP) idle=0ba/1/4611686018427387906
>> >> >> softirq=57641/57641 fqs=31151
>> >> >> (t=125002 jiffies g=31656 c=31655 q=910)
>> >> >>
>> >> >> INFO: rcu_sched self-detected stall on CPU
>> >> >> INFO: rcu_sched detected stalls on CPUs/tasks:
>> >> >> 0-....: (125000 ticks this GP) idle=49a/1/4611686018427387906
>> >> >> softirq=65194/65194 fqs=31231
>> >> >> 0-....: (125000 ticks this GP) idle=49a/1/4611686018427387906
>> >> >> softirq=65194/65194 fqs=31231
>> >> >> (t=125002 jiffies g=34421 c=34420 q=1119)
>> >> >> (detected by 1, t=125002 jiffies, g=34421, c=34420, q=1119)
>> >> >>
>> >> >>
>> >> >> and then there is an unintelligible mess of 2 reports. Such crashes go
>> >> >> to trash bin, because we can't even say which function hanged. It
>> >> >> seems that in all cases 2 different rcu stall detection facilities
>> >> >> race with each other. Is it possible to make them not race?
>> >> >
>> >> > How about the following (untested, not for mainline) patch? It suppresses
>> >> > all but the "main" RCU flavor, which is rcu_sched for !PREEMPT builds and
>> >> > rcu_preempt otherwise. Either way, this is the RCU flavor corresponding
>> >> > to synchronize_rcu(). This works well in the common case where there
>> >> > is almost always an RCU grace period in flight.
>> >> >
>> >> > One reason that this patch is not for mainline is that I am working on
>> >> > merging the RCU-bh, RCU-preempt, and RCU-sched flavors into one thing,
>> >> > at which point there won't be any races. But that might be a couple
>> >> > merge windows away from now.
>> >> >
>> >> > Thanx, Paul
>> >> >
>> >> > ------------------------------------------------------------------------
>> >> >
>> >> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> >> > index 381b47a68ac6..31f7818f2d63 100644
>> >> > --- a/kernel/rcu/tree.c
>> >> > +++ b/kernel/rcu/tree.c
>> >> > @@ -1552,7 +1552,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>> >> > struct rcu_node *rnp;
>> >> >
>> >> > if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
>> >> > - !rcu_gp_in_progress(rsp))
>> >> > + !rcu_gp_in_progress(rsp) || rsp != rcu_state_p)
>> >> > return;
>> >> > rcu_stall_kick_kthreads(rsp);
>> >> > j = jiffies;
>> >>
>> >> But doesn't they both relate to the same rcu flavor? They both say
>> >> rcu_sched. I assumed that the difference is "self-detected" vs "on
>> >> CPUs/tasks", i.e. on the current CPU vs on other CPUs.
>> >
>> > Right you are!
>> >
>> > One approach would be to increase the value of RCU_STALL_RAT_DELAY,
>> > which is currently two jiffies to (say) 20 jiffies. This is in
>> > kernel/rcu/tree.h. But this would fail on a sufficiently overloaded
>> > system -- and the failure of the two-jiffy delay is a bit of a surprise,
>> > given interrupts disabled and all that. Are you by any chance loaded
>> > heavily enough to see vCPU preemption?
>> >
>> > I could avoid at least some of these timing issues instead using cmpxchg()
>> > on ->jiffies_stall to allow only one CPU in, but leave the non-atomic
>> > update to discourage overly long stall prints from running into the
>> > next one. This is not perfect, either, and is roughly equivalent to
>> > setting RCU_STALL_RAT_DELAY to many second's worth of jiffies, but
>> > avoiding that minute's delay. But it should get rid of the duplication
>> > in almost all cases, though it could allow a stall warning to overlap
>> > with a later stall warning for that same grace period. Which can
>> > already happen anyway. Also, a tens-of-seconds vCPU preemption can
>> > still cause concurrent stall warnings, but if that is happening to you,
>> > the concurrent stall warnings are probably the least of your problems.
>> > Besides, we do need at least one CPU to actually report the stall, which
>> > won't happen if that CPU's vCPU is indefinitely preempted. So there is
>> > only so much I can do about that particular corner case.
>> >
>> > So how does the following (untested) patch work for you?
>>
>> Looks good to me.
>>
>> We run on VMs, so we can well have vCPU preemption.
>
> Very good! Please do get me a Tested-by when you get to that point.

Unfortunately I don't have a good way to test it until it's submitted
upstream. While we are seeing thousands of such instances, they happen
episodically on a farm of test machines. But they are still harmful,
especially when the system tries to reproduce a bug, because it's
mid-way through and thinks it got a hook, but then suddenly boom! it
gets some mess that it can't parse and now it does not know if it's
still the same bug, or maybe a different bug triggered by the same
program, so it does not know how to properly attribute the reproducer.
You can see these cases as they happen here (under report/log links in
the table):
https://syzkaller.appspot.com/bug?id=d5bc3e0c66d200d72216ab343a67c4327e4a3452
When the patch is submitted, the rate should go down.

> Thanx, Paul
>
>> > ------------------------------------------------------------------------
>> >
>> > commit 6a5ab1e68f8636d8823bb5a9aee35fc44c2be866
>> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> > Date: Mon Apr 9 11:04:46 2018 -0700
>> >
>> > rcu: Exclude near-simultaneous RCU CPU stall warnings
>> >
>> > There is a two-jiffy delay between the time that a CPU will self-report
>> > an RCU CPU stall warning and the time that some other CPU will report a
>> > warning on behalf of the first CPU. This has worked well in the past,
>> > but on busy systems, it is possible for the two warnings to overlap,
>> > which makes interpreting them extremely difficult.
>> >
>> > This commit therefore uses a cmpxchg-based timing decision that
>> > allows only one report in a given one-minute period (assuming default
>> > stall-warning Kconfig parameters). This approach will of course fail
>> > if you are seeing minute-long vCPU preemption, but in that case the
>> > overlapping RCU CPU stall warnings are the least of your worries.
>> >
>> > Reported-by: Dmitry Vyukov <dvyukov@google.com>
>> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> >
>> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> > index 381b47a68ac6..b7246bcbf633 100644
>> > --- a/kernel/rcu/tree.c
>> > +++ b/kernel/rcu/tree.c
>> > @@ -1429,8 +1429,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
>> > raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>> > return;
>> > }
>> > - WRITE_ONCE(rsp->jiffies_stall,
>> > - jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
>> > raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>> >
>> > /*
>> > @@ -1481,6 +1479,10 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
>> > sched_show_task(current);
>> > }
>> > }
>> > + /* Rewrite if needed in case of slow consoles. */
>> > + if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
>> > + WRITE_ONCE(rsp->jiffies_stall,
>> > + jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
>> >
>> > rcu_check_gp_kthread_starvation(rsp);
>> >
>> > @@ -1525,6 +1527,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
>> > rcu_dump_cpu_stacks(rsp);
>> >
>> > raw_spin_lock_irqsave_rcu_node(rnp, flags);
>> > + /* Rewrite if needed in case of slow consoles. */
>> > if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
>> > WRITE_ONCE(rsp->jiffies_stall,
>> > jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
>> > @@ -1548,6 +1551,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>> > unsigned long gpnum;
>> > unsigned long gps;
>> > unsigned long j;
>> > + unsigned long jn;
>> > unsigned long js;
>> > struct rcu_node *rnp;
>> >
>> > @@ -1586,14 +1590,17 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>> > ULONG_CMP_GE(gps, js))
>> > return; /* No stall or GP completed since entering function. */
>> > rnp = rdp->mynode;
>> > + jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
>> > if (rcu_gp_in_progress(rsp) &&
>> > - (READ_ONCE(rnp->qsmask) & rdp->grpmask)) {
>> > + (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
>> > + cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
>> >
>> > /* We haven't checked in, so go dump stack. */
>> > print_cpu_stall(rsp);
>> >
>> > } else if (rcu_gp_in_progress(rsp) &&
>> > - ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) {
>> > + ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
>> > + cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
>> >
>> > /* They had a few time units to dump stack, so complain. */
>> > print_other_cpu_stall(rsp, gpnum);
>> >
>>
>