"Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:
> On Tue, Mar 23, 2021 at 06:29:35PM +0100, Toke Høiland-Jørgensen wrote:
>> "Paul E. McKenney" <paulmck@xxxxxxxxxx> writes:
>>
>> > On Tue, Mar 23, 2021 at 01:26:36PM +0100, Toke Høiland-Jørgensen wrote:
>> >> Hi Paul
>> >>
>> >> Magnus and I have been debugging an issue where close() on a bpf_link
>> >> file descriptor would hang indefinitely when the system was under load
>> >> on a kernel compiled with CONFIG_PREEMPT=y, and it seems to be related
>> >> to synchronize_rcu_tasks(), so I'm hoping you can help us with it.
>> >>
>> >> The issue is triggered reliably by loading up a system with network
>> >> traffic (causing 100% softirq CPU load on one or more cores), and then
>> >> attaching an freplace bpf_link and closing it again. The close() will
>> >> hang until the network traffic load is lowered.
>> >>
>> >> Digging further, it appears that the hang happens in
>> >> synchronize_rcu_tasks(), as seen by running a bpftrace script like:
>> >>
>> >> bpftrace -e 'kprobe:synchronize_rcu_tasks { @start = nsecs; printf("enter\n"); } kretprobe:synchronize_rcu_tasks { printf("exit after %d ms\n", (nsecs - @start) / 1000000); }'
>> >> Attaching 2 probes...
>> >> enter
>> >> exit after 54 ms
>> >> enter
>> >> exit after 3249 ms
>> >>
>> >> (the two enter/exit pairs are, respectively, from an unloaded system,
>> >> and from a loaded system where I stopped the network traffic after a
>> >> couple of seconds).
>> >>
>> >> The call to synchronize_rcu_tasks() happens in bpf_trampoline_put():
>> >>
>> >> https://elixir.bootlin.com/linux/latest/source/kernel/bpf/trampoline.c#L376
>> >>
>> >> And because it does this while holding trampoline_mutex, even deferring
>> >> the put to a worker (as a previously applied-then-reverted patch did[0])
>> >> doesn't help: that'll fix the initial hang on close(), but any
>> >> subsequent use of BPF trampolines will then be blocked because of the
>> >> mutex.
>> >>
>> >> Also, if I just keep the network traffic running I will eventually get a
>> >> kernel panic with:
>> >>
>> >> kernel:[44348.426312] Kernel panic - not syncing: hung_task: blocked tasks
>> >>
>> >> I've created a reproducer for the issue here:
>> >> https://github.com/xdp-project/bpf-examples/tree/master/bpf-link-hang
>> >>
>> >> To compile simply do this (needs a recent llvm/clang for compiling the BPF program):
>> >>
>> >> $ git clone --recurse-submodules https://github.com/xdp-project/bpf-examples
>> >> $ cd bpf-examples/bpf-link-hang
>> >> $ make
>> >> $ ./sudo bpf-link-hang
>> >>
>> >> you'll need to load up the system to trigger the hang; I'm using pktgen
>> >> from a separate machine to do this.
>> >>
>> >> My question is, of course, as ever, What Is To Be Done? Is it expected
>> >> that synchronize_rcu_tasks() can hang indefinitely on a PREEMPT system,
>> >> or can this be fixed? And if it is expected, how can the BPF code be
>> >> fixed so it doesn't deadlock because of this?
>> >>
>> >> Hoping you can help us with this - many thanks in advance! :)
>> >
>> > Let me start with the usual question... Is the network traffic intense
>> > enough that one of the CPUs might remain in a loop handling softirqs
>> > indefinitely?
>>
>> Yup, I'm pegging all CPUs in softirq:
>>
>> $ mpstat -P ALL 1
>> [...]
>> 18:26:52 CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
>> 18:26:53 all 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>> 18:26:53 0 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>> 18:26:53 1 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>> 18:26:53 2 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>> 18:26:53 3 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>> 18:26:53 4 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>> 18:26:53 5 0.00 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00
>>
>> > If so, does the (untested, probably does not build) patch below help?
>>
>> Doesn't appear to, no. It builds fine, but I still get:
>>
>> Attaching 2 probes...
>> enter
>> exit after 8480 ms
>>
>> (that was me interrupting the network traffic again)
>
> Is your kernel properly shifting from back-of-interrupt softirq processing
> to ksoftirqd under heavy load? If not, my patch will not have any
> effect.
Seems to be - this is from top:
12 root 20 0 0 0 0 R 99.3 0.0 0:43.64 ksoftirqd/0
24 root 20 0 0 0 0 R 99.3 0.0 0:43.62 ksoftirqd/2
34 root 20 0 0 0 0 R 99.3 0.0 0:43.64 ksoftirqd/4
39 root 20 0 0 0 0 R 99.3 0.0 0:43.65 ksoftirqd/5
19 root 20 0 0 0 0 R 99.0 0.0 0:43.63 ksoftirqd/1
29 root 20 0 0 0 0 R 99.0 0.0 0:43.63 ksoftirqd/3
Any other ideas? :)
(And thanks for taking a look, BTW!)
-Toke
