Re: [PATCH] perf probe: Fix failure to probe events on arm

[hekuang]

hi, Arnaldo

On 06/15/2015 10:49 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jun 15, 2015 at 08:06:53AM +0000, He Kuang escreveu:
> > Fix failure to probe events on arm, problem is introduced by commit
> > 5a51fcd1f30c ("perf probe: Skip kernel symbols which is out of
> > .text"). For some architectures, label '_etext' is not in the .text
> > section(in .notes section for arm/arm64). Label out of .text section is
> > not loaded as symbols and we got a zero value when look up its address,
> > which causes all events be wrongly skiped.
> >
> > This patch uses kernel map->end when failed to get the address of
> > '_etext' and fixes the problem.
> Masami, can't we always use map->end then? Can you please take a look at
> this patch and ack/nack it?
>
> - Arnaldo

I think _etext is more accurate than kernel map->end, because
__map_groups__fixup_end() is called at the end of
dso__load_sym(), which fixes map->end to next_map->start.

Comparative result as this:
  etext_addr=ffffffff819a1b85, map->end=ffffffff81ff1000.

So if possible, we should use _etext.

Thanks.
>   
> > Problem can be reproduced on arm as following:
> >
> >    # perf probe --add='generic_perform_write'
> >    generic_perform_write+0 is out of .text, skip it.
> >    Probe point 'generic_perform_write' not found.
> >      Error: Failed to add events. Reason: No such file or directory (Code: -2)
> >
> > After this patch:
> >
> >    # perf probe --add='generic_perform_write'
> >    Added new event:
> >      probe:generic_perform_write (on generic_perform_write)
> >
> >    You can now use it in all perf tools, such as:
> >
> >      perf record -e probe:generic_perform_write -aR sleep 1
> >
> > Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
> > ---
> >   tools/perf/util/probe-event.c | 16 +++++++++++++++-
> >   1 file changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> > index daa24a2..ee26961 100644
> > --- a/tools/perf/util/probe-event.c
> > +++ b/tools/perf/util/probe-event.c
> > @@ -575,8 +575,22 @@ static int post_process_probe_trace_events(struct probe_trace_event *tevs,
> >   		pr_warning("Relocated base symbol is not found!\n");
> >   		return -EINVAL;
> >   	}
> > -	/* Get the address of _etext for checking non-probable text symbol */
> > +	/* Get the address of _etext for checking non-probable text symbol,
> > +	   for some architectures (e.g. arm, arm64), _etext is out of .text
> > +	   section and not loaded as symbols, use kernel map->end instead.
> > +	 */
> >   	etext_addr = kernel_get_symbol_address_by_name("_etext", false);
> > +	if (etext_addr == 0) {
> > +		struct map *map;
> > +
> > +		map = kernel_get_module_map(NULL);
> > +		if (!map) {
> > +			pr_err("Failed to get a map for kernel\n");
> > +			return -EINVAL;
> > +		}
> > +
> > +		etext_addr = map->end;
> > +	}
> >   
> >   	for (i = 0; i < ntevs; i++) {
> >   		if (tevs[i].point.address && !tevs[i].point.retprobe) {
> > --
> > 1.8.5.2
> --
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