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

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Message-ID: <20150616133937.GB10374@kernel.org>
Date:	Tue, 16 Jun 2015 10:39:37 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	He Kuang <hekuang@...wei.com>, a.p.zijlstra@...llo.nl,
	mingo@...hat.com, namhyung@...nel.org, wangnan0@...wei.com,
	linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] perf probe: Fix failure to probe events on arm

Em Tue, Jun 16, 2015 at 03:05:17PM +0900, Masami Hiramatsu escreveu:
> On 2015/06/15 23:49, 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?
> 
> Yeah, it looks good to me, and we finally will replace "_etext" with it :)
> 
> ----
> void map__fixup_end(struct map *map)
> {
>         struct rb_root *symbols = &map->dso->symbols[map->type];
>         struct rb_node *nd = rb_last(symbols);
>         if (nd != NULL) {
>                 struct symbol *sym = rb_entry(nd, struct symbol, rb_node);
>                 map->end = sym->end;
>         }
> }
> ----
> So, the map->end shows the end address of the symbols.
> 
> Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>

I can and will apply it, but the question remains, if this approach is
good then when should we bother looking at "_etext" at all, i.e. why not
just use map->end all the time?

I have not thought this thru, this is just based on this ``its ok to use
map->end if "_etext" is not found'' idea that He proposed and you acked.

- Arnaldo
 
> 
> Thanks!
> 
> > 
> > - Arnaldo
> > 
> >  
> >> 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@...wei.com>
> >> ---
> >>  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
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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> > Please read the FAQ at  http://www.tux.org/lkml/
> > 
> 
> 
> -- 
> Masami HIRAMATSU
> Linux Technology Research Center, System Productivity Research Dept.
> Center for Technology Innovation - Systems Engineering
> Hitachi, Ltd., Research & Development Group
> E-mail: masami.hiramatsu.pt@...achi.com
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