linux-kernel - Re: [PATCHv2 01/15] perf tools: Fix -n option

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Date:   Mon, 3 Jul 2017 18:22:28 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Thomas Richter <tmricht@...ux.vnet.ibm.com>
Subject: Re: [PATCHv2 01/15] perf tools: Fix -n option

On Mon, Jul 03, 2017 at 01:10:04PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jul 03, 2017 at 05:16:54PM +0200, Jiri Olsa escreveu:
> > On Mon, Jul 03, 2017 at 11:57:39AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Mon, Jul 03, 2017 at 04:50:16PM +0200, Jiri Olsa escreveu:
> > > > The kernel fails to add a non sampling event event
> > > > with having precise_ip set.
> > > > 
> > > > Make sure the precise_ip is 0 when using -n option
> > > > to create non sampling event in record.
> > > 
> > > Ok, but how could that happen? What was the scenario? Which command? Can
> > > you provide a command line that causes the problem?
> > 
> > [jolsa@...va perf]$ ./perf record -e cycles:ppp -n ls
> > Lowering default frequency rate to 3000.
> > Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate.
> > Error:
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (cycles:pppu).
> > /bin/dmesg may provide additional information.
> > No CONFIG_PERF_EVENTS=y kernel support configured?
> > 
> > v2 attached, thanks
> 
> Ok, so the user asks for --no-samples but at the same time asks for
> precision equal to :ppp, we should stop right there and warn the user
> that that is not possible, instead of silently dropping off what the
> user explicitely asked.

well I take this option more like debug/devel one.. you have
failing command line and want to investigate the kernel state
without having samples generated under your fingers

can't see why would normal user need it

> 
> I'm cooking a few patches to allow that, then we can apply your patch,
> that, with the current set of users will never kick in :-)


ok

jirka