linux-kernel - Re: [PATCH] tools/perf probe: Print a hint if adding a probe fails

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Date:   Fri, 18 Jun 2021 10:15:42 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH] tools/perf probe: Print a hint if adding a probe fails

Em Thu, Jun 10, 2021 at 07:29:26PM +0900, Masami Hiramatsu escreveu:
> Hi Naveen,
> 
> On Thu, 10 Jun 2021 15:14:42 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
> > Adding a probe can fail in a few scenarios. perf already checks for the
> > address in the kprobe blacklist. However, the address could still be a
> > jump label, or have a BUG_ON(). In such cases, it isn't always evident
> > why adding the probe failed. Add a hint so that the user knows how to
> > proceed.
> > 
> 
> Thanks for the report.
> 
> Since now there is <tracefs>/error_log, if you see any errors in registering
> probe-events, perf probe should dump the error_log for the hint message.
> Also, kprobes should return the correct different error code for each
> errors.

Was there any followup on this? I think we should do as Masami suggests,
Naveen, could you do it?

Thanks,

- Arnaldo
 
> Thank you,
> 
> > Reported-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> > ---
> >  tools/perf/builtin-probe.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/perf/builtin-probe.c b/tools/perf/builtin-probe.c
> > index 6b150756677014..ff9f3fdce600dd 100644
> > --- a/tools/perf/builtin-probe.c
> > +++ b/tools/perf/builtin-probe.c
> > @@ -352,8 +352,11 @@ static int perf_add_probe_events(struct perf_probe_event *pevs, int npevs)
> >  	}
> >  
> >  	ret = apply_perf_probe_events(pevs, npevs);
> > -	if (ret < 0)
> > +	if (ret < 0) {
> > +		pr_info("Hint: Check dmesg to understand reason for probe failure.\n"
> > +			"      Consider probing at the next/previous instruction.\n");
> >  		goto out_cleanup;
> > +	}
> >  
> >  	for (i = k = 0; i < npevs; i++)
> >  		k += pevs[i].ntevs;
> > 
> > base-commit: 0808b3d5b7514dc856178dbc509929329bbf301d
> > -- 
> > 2.31.1
> > 
> 
> 
> -- 
> Masami Hiramatsu <mhiramat@...nel.org>

-- 

- Arnaldo