On Tue, Sep 5, 2017 at 7:40 PM Y Song via iovisor-dev <
iovisor-dev@lists.iovisor.org> wrote:
> On Tue, Sep 5, 2017 at 7:43 AM, Nair, Reena via iovisor-dev
> <iovisor-dev@lists.iovisor.org> wrote:
> > Hi,
> >
> >
> > I was trying to retrieve stack traces from tracepoint probe by modifying
> > offcputime.py
> >
> >
> > The original code is in blue and the changed code is in yellow.
> >
> >
> > //int oncpu(struct pt_regs *ctx, struct task_struct *prev) {
> > TRACEPOINT_PROBE(sched, sched_switch){
> >
> > //u32 pid = prev->pid;
> > //u32 tgid = prev->tgid;
> >
> > u32 pid = args->prev_pid, tgid;
> > u64 ts, *tsp;
> > // record previous thread sleep time
> > if ((THREAD_FILTER) && (STATE_FILTER)) {
> > ts = bpf_ktime_get_ns();
> > start.update(&pid, &ts);
> > }
> > // get the current thread's start time
> > //pid = bpf_get_current_pid_tgid();
> >
> > pid = args->next_pid;
> > tgid = bpf_get_current_pid_tgid() >> 32;
> >
> > ...
> > ...
> > ...
> >
> > I also replaced 'ctx' with 'args'
> >
> > kernel_stack_get = "stack_traces.get_stackid(args, BPF_F_REUSE_STACKID)"
> > user_stack_get = \
> > "stack_traces.get_stackid(args, BPF_F_REUSE_STACKID |
> BPF_F_USER_STACK)"
>
> This won't work. get_stackid needs pt_regs so that it can walk through
> the frame to
> get stacks. Here for tracepoint, "args" is the tracepoint structure.
>
It works for me in the 'trace' tool, e.g.:
# /usr/share/bcc/tools/trace -K t:sched:sched_process_fork
PID TID COMM FUNC
1632 1632 sshd sched_process_fork
_do_fork+0x1e9 [kernel]
SyS_clone+0x19 [kernel]
do_syscall_64+0x5e [kernel]
return_from_SYSCALL_64+0x0 [kernel]
The tool uses stacks.get_stack_id(args, ...) exactly like the code above.
Indeed it looks like the patch linked below is responsible for enabling it
-- unless I'm missing something.
>
> >
> > However, no stack traces, kernel/ user, were returned. Is it not
> possible to
> > get stack traces from tracepoint probes?
> >
> > I came across this link:
> > https://patchwork.kernel.org/patch/8747201/
> >
> > Do we have any functions to access the pt_regs from tracepoint probes? I
> am
> > running the probes in kernel version 4.8.
> >
> > Thanks,
> > Reena.
> >
> >
> >
> >
> > _______________________________________________
> > iovisor-dev mailing list
> > iovisor-dev@lists.iovisor.org
> > https://lists.iovisor.org/mailman/listinfo/iovisor-dev
> >
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