Re: latency histogram with BPF

[Alexei Starovoitov]

On 6/12/15 7:33 AM, Daniel Wagner wrote:
> On 06/12/2015 08:12 AM, Daniel Wagner wrote:
> > On 06/12/2015 12:08 AM, Alexei Starovoitov wrote:
> > > On 6/11/15 12:25 AM, Daniel Wagner wrote:
> > > If you have any suggestions on where to look, I'm all ears.
> > > My stack traces look like:
> > > Running with 10*40 (== 400) tasks.
> > > [   12.032571] kernel BUG at ../mm/slub.c:3413!
>
> I hit this as well.
>
> After looking and playing around for while I think I found the source of
> the problem: The path from the BPF program into the hash table code is
> triggering the crash.
>
> Attaching kprobes to trace_preempt_[on|off] works fine. Empty BPF
> programs connected to the probes is no problem as well. So I changed the
> BPF program to use only arrays instead of hash tables. No crash anymore.

yes. I've tried that too. arrays work fine indeed.

> I suspect the hash table code will call trace_preempt_[off|on]
> eventually and that is not going to fly.

The recursive calls into bpf programs are detected and prevented.
That's ok. I've tested attaching kprobes to kmalloc/kfree and
from the program do hash_map->update_elem->kmalloc which triggers
recursive call into the same program. All works fine.
There is something else here.

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