> On May 19, 2019, at 11:07 AM, Kairui Song <kasong@xxxxxxxxxx> wrote:
>
> On Sat, May 18, 2019 at 5:48 AM Song Liu <songliubraving@xxxxxx> wrote:
>>
>>
>>
>>> On May 17, 2019, at 2:06 PM, Alexei Starovoitov <ast@xxxxxx> wrote:
>>>
>>> On 5/17/19 11:40 AM, Song Liu wrote:
>>>> +Alexei, Daniel, and bpf
>>>>
>>>>> On May 17, 2019, at 2:10 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>> On Fri, May 17, 2019 at 04:15:39PM +0800, Kairui Song wrote:
>>>>>> Hi, I think the actual problem is that bpf_get_stackid_tp (and maybe
>>>>>> some other bfp functions) is now broken, or, strating an unwind
>>>>>> directly inside a bpf program will end up strangely. It have following
>>>>>> kernel message:
>>>>>
>>>>> Urgh, what is that bpf_get_stackid_tp() doing to get the regs? I can't
>>>>> follow.
>>>>
>>>> I guess we need something like the following? (we should be able to
>>>> optimize the PER_CPU stuff).
>>>>
>>>> Thanks,
>>>> Song
>>>>
>>>>
>>>> diff --git i/kernel/trace/bpf_trace.c w/kernel/trace/bpf_trace.c
>>>> index f92d6ad5e080..c525149028a7 100644
>>>> --- i/kernel/trace/bpf_trace.c
>>>> +++ w/kernel/trace/bpf_trace.c
>>>> @@ -696,11 +696,13 @@ static const struct bpf_func_proto bpf_perf_event_output_proto_tp = {
>>>> .arg5_type = ARG_CONST_SIZE_OR_ZERO,
>>>> };
>>>>
>>>> +static DEFINE_PER_CPU(struct pt_regs, bpf_stackid_tp_regs);
>>>> BPF_CALL_3(bpf_get_stackid_tp, void *, tp_buff, struct bpf_map *, map,
>>>> u64, flags)
>>>> {
>>>> - struct pt_regs *regs = *(struct pt_regs **)tp_buff;
>>>> + struct pt_regs *regs = this_cpu_ptr(&bpf_stackid_tp_regs);
>>>>
>>>> + perf_fetch_caller_regs(regs);
>>>
>>> No. pt_regs is already passed in. It's the first argument.
>>> If we call perf_fetch_caller_regs() again the stack trace will be wrong.
>>> bpf prog should not see itself, interpreter or all the frames in between.
>>
>> Thanks Alexei! I get it now.
>>
>> In bpf_get_stackid_tp(), the pt_regs is get by dereferencing the first field
>> of tp_buff:
>>
>> struct pt_regs *regs = *(struct pt_regs **)tp_buff;
>>
>> tp_buff points to something like
>>
>> struct sched_switch_args {
>> unsigned long long pad;
>> char prev_comm[16];
>> int prev_pid;
>> int prev_prio;
>> long long prev_state;
>> char next_comm[16];
>> int next_pid;
>> int next_prio;
>> };
>>
>> where the first field "pad" is a pointer to pt_regs.
>>
>> @Kairui, I think you confirmed that current code will give empty call trace
>> with ORC unwinder? If that's the case, can we add regs->ip back? (as in the
>> first email of this thread.
>>
>> Thanks,
>> Song
>>
>
> Hi thanks for the suggestion, yes we can add it should be good an idea
> to always have IP when stack trace is not available.
> But stack trace is actually still broken, it will always give only one
> level of stacktrace (the IP).
I think this is still the best fix/workaround here? And only one level
of stack trace should be OK for tracepoint?
Thanks,
Song
>
> --
> Best Regards,
> Kairui Song
