Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth via sysctl

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	David Ahern <dsahern@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Alexei Starovoitov <ast@kernel.org>,
	He Kuang <hekuang@huawei.com>, Jiri Olsa <jolsa@redhat.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Milian Wolff <milian.wolff@kdab.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Stephane Eranian <eranian@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vince Weaver <vincent.weaver@maine.edu>,
	Wang Nan <wangnan0@huawei.com>, Zefan Li <lizefan@huawei.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth via sysctl
Date: Wed, 27 Apr 2016 09:56:02 -0300	[thread overview]
Message-ID: <20160427125602.GN11033@kernel.org> (raw)
In-Reply-To: <CAE40pdcd+mXur=bkHjN8Fb7SgVYNjDbR30rRS=LrqCknAJa9ig@mail.gmail.com>

Em Tue, Apr 26, 2016 at 02:58:41PM -0700, Brendan Gregg escreveu:
> On Tue, Apr 26, 2016 at 2:05 PM, Arnaldo Carvalho de Melo
> <arnaldo.melo@gmail.com> wrote:
> > Em Tue, Apr 26, 2016 at 01:02:34PM -0700, Brendan Gregg escreveu:
> >> On Mon, Apr 25, 2016 at 5:49 PM, Brendan Gregg <brendan.d.gregg@gmail.com> wrote:
> >> > On Mon, Apr 25, 2016 at 5:47 PM, Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote:
> >> >> Em Mon, Apr 25, 2016 at 05:44:00PM -0700, Alexei Starovoitov escreveu:
> >> >>> yep :)
> >> >>> hopefully Brendan can give it another spin.
> >> >>
> >> >> Agreed, and I'm calling it a day anyway, Brendan, please consider
> >> >> retesting, thanks,
> >> >
> >> > Will do, thanks!
> >>
> >> Looks good.
> >>
> >> I started with max depth = 512, and even that was still truncated, and
> >> had to profile again at 1024 to capture the full stacks. Seems to
> >> generally match the flame graph I generated with V1, which made me
> >> want to check that I'm running the new patch, and am:
> >>
> >> # grep six_hundred_forty_kb /proc/kallsyms
> >> ffffffff81c431e0 d six_hundred_forty_kb
> >>
> >> I was mucking around and was able to get "corrupted callchain.
> >> skipping..." errors, but these look to be expected -- that was
> >
> > Yeah, thanks for testing!
> >
> > And since you talked about userspace without frame pointers, have you
> > played with '--call-graph lbr'?
> 
> Not really. Isn't it only 16 levels deep max?

Yeah, stoopid me :-\

> Most of our Linux is Xen guests (EC2), and I'd have to check if the
> MSRs are available for LBR (perf record --call-graph lbr ... returns
> "The sys_perf_event_open() syscall returned with 95 (Operation not

That is because it is only accepted for PERF_TYPE_HARDWARE on x86, i.e.
it retunrs EOPNOTSUPP for all the other cases, like for
PERF_TYPE_SOFTWARE counters, like "cpu-clock"

> supported) for event (cpu-clock).", so I'd guess not, although many
> other MSRs are exposed).
> 
> BTS seemed more promising (deeper stacks), and there's already Xen
> support for it (need to boot the Xen host with vpmu=bts, preferably
> vpmu=bts,arch for some PMCs as well :).

Yeah, worth a look, I guess, but doesn't look like a low hanging fruit
tho.

- Arnaldo