Re: [PATCH v9 3.2 7/9] tracing: uprobes trace_event interface

[Jiri Olsa <jolsa@redhat.com>]

On Mon, Jan 16, 2012 at 08:15:38PM +0530, Srikar Dronamraju wrote:
> > 
> > I've tested following event:
> >         echo "p:probe_libc/free /lib64/libc-2.13.so:0x7a4f0 %ax" > ./uprobe_events
> > 
> > and commands like:
> >         perf record -a -e probe_libc:free  --filter "common_pid == 1127"
> >         perf record -e probe_libc:free --filter "arg1 == 0xa" ls
> > 
> > got me proper results.
> > 
> 
> Okay thanks for the inputs.
> 
> > thanks,
> > jirka
> > 
> > ---
> > The preemption needs to be disabled when submitting data into perf.
> 
> I actually looked at other places where perf_trace_buf_prepare and
> perf_trace_buf_submit are being called. for example perf_syscall_enter
> and perf_syscall_exit both call the above routines and they didnt seem
> to be called with premption disabled. Is that the way perf probe is
> called in our case that needs us to call pre-emption here? Did you see a
> case where calling these without preemption disabled caused a problem?

the perf_trace_buf_prepare touches per cpu variables,
hence the preemption disabling

the perf_trace_buf_prepare code is used by syscalls,
kprobes, and trace events

- both syscalls and trace events are implemented by
  tracepoints which disable preemption before calling the probe
  (see __DO_TRACE macro in include/linux/tracepoint.h)

- kprobes disable preemption as well
  (kprobe_handler in arch/x86/kernel/kprobes.c)
  haven't checked the optimalized kprobes,
  but should be the same case

jirka

> 
> 
> > ---
> >  kernel/trace/trace_uprobe.c |    6 +++++-
> >  1 files changed, 5 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > index af29368..4d3857c 100644
> > --- a/kernel/trace/trace_uprobe.c
> > +++ b/kernel/trace/trace_uprobe.c
> > @@ -653,9 +653,11 @@ static void uprobe_perf_func(struct trace_uprobe *tp, struct pt_regs *regs)
> >  		     "profile buffer not large enough"))
> >  		return;
> > 
> > +	preempt_disable();
> > +
> >  	entry = perf_trace_buf_prepare(size, call->event.type, regs, &rctx);
> >  	if (!entry)
> > -		return;
> > +		goto out;
> > 
> >  	entry->ip = get_uprobe_bkpt_addr(task_pt_regs(current));
> >  	data = (u8 *)&entry[1];
> > @@ -665,6 +667,8 @@ static void uprobe_perf_func(struct trace_uprobe *tp, struct pt_regs *regs)
> > 
> >  	head = this_cpu_ptr(call->perf_events);
> >  	perf_trace_buf_submit(entry, size, rctx, entry->ip, 1, regs, head);
> > + out:
> > +	preempt_enable();
> >  }
> >  #endif	/* CONFIG_PERF_EVENTS */
> > 
> 
> -- 
> Thanks and Regards
> Srikar
>