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

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Jiri Olsa <jolsa@redhat.com> [2012-01-16 16:33:27]:

> 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
> 

Okay, thanks for the clarification, I will pick the fix.

--
Thanks and Regards
Srikar

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