linux-kernel - Re: [REGRESSION] Perf (userspace) broken on big.LITTLE systems since v6.5

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Date:   Wed, 22 Nov 2023 15:49:18 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Hector Martin <marcan@...can.st>, Ian Rogers <irogers@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        James Clark <james.clark@....com>,
        linux-perf-users@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Asahi Linux <asahi@...ts.linux.dev>
Subject: Re: [REGRESSION] Perf (userspace) broken on big.LITTLE systems since
 v6.5

On Wed, Nov 22, 2023 at 10:06:23AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Nov 22, 2023 at 12:23:27PM +0900, Hector Martin escreveu:
> > On 2023/11/22 1:38, Ian Rogers wrote:
> > > On Tue, Nov 21, 2023 at 8:15 AM Mark Rutland <mark.rutland@....com> wrote:
> > >> On Tue, Nov 21, 2023 at 08:09:37AM -0800, Ian Rogers wrote:
> > >>> On Tue, Nov 21, 2023 at 8:03 AM Mark Rutland <mark.rutland@....com> wrote:
> > >>>> On Tue, Nov 21, 2023 at 07:46:57AM -0800, Ian Rogers wrote:
> > >>>>> On Tue, Nov 21, 2023 at 7:40 AM Mark Rutland <mark.rutland@....com> wrote:
> > >>>>>> On Tue, Nov 21, 2023 at 03:24:25PM +0000, Marc Zyngier wrote:
> > >>>>>>> On Tue, 21 Nov 2023 13:40:31 +0000,
> > >>>>>>> Marc Zyngier <maz@...nel.org> wrote:
> > >>>>>>>>
> > >>>>>>>> [Adding key people on Cc]
> > >>>>>>>>
> > >>>>>>>> On Tue, 21 Nov 2023 12:08:48 +0000,
> > >>>>>>>> Hector Martin <marcan@...can.st> wrote:
> > >>>>>>>>>
> > >>>>>>>>> Perf broke on all Apple ARM64 systems (tested almost everything), and
> > >>>>>>>>> according to maz also on Juno (so, probably all big.LITTLE) since v6.5.
> > >>>>>>>>
> > >>>>>>>> I can confirm that at least on 6.7-rc2, perf is pretty busted on any
> > >>>>>>>> asymmetric ARM platform. It isn't clear what criteria is used to pick
> > >>>>>>>> the PMU, but nothing works anymore.
> > >>>>>>>>
> > >>>>>>>> The saving grace in my case is that Debian still ships a 6.1 perftool
> > >>>>>>>> package, but that's obviously not going to last.
> > >>>>>>>>
> > >>>>>>>> I'm happy to test potential fixes.
> > >>>>>>>
> > >>>>>>> At Mark's request, I've dumped a couple of perf (as of -rc2) runs with
> > >>>>>>> -vvv.  And it is quite entertaining (this is taskset to an 'icestorm'
> > >>>>>>> CPU):
> > >>>>>>
> > >>>>>> IIUC the tool is doing the wrong thing here and overriding explicit
> > >>>>>> ${pmu}/${event}/ events with PERF_TYPE_HARDWARE events rather than events using
> > >>>>>> that ${pmu}'s type and event namespace.
> > >>>>>>
> > >>>>>> Regardless of the *new* ABI that allows PERF_TYPE_HARDWARE events to be
> > >>>>>> targetted to a specific PMU, it's semantically wrong to rewrite events like
> > >>>>>> this since ${pmu}/${event}/ is not necessarily equivalent to a similarly-named
> > >>>>>> PERF_COUNT_HW_${EVENT}.
> > >>>>>
> > >>>>> If you name a PMU and an event then the event should only be opened on
> > >>>>> that PMU, 100% agree. There's a bunch of output, but when the legacy
> > >>>>> cycles event is opened it appears to be because it was explicitly
> > >>>>> requested.
> > >>>>
> > >>>> I think you've missed that the named PMU events are being erreously transformed
> > >>>> into PERF_TYPE_HARDWARE events. Look at the -vvv output, e.g.
> > >>>>
> > >>>>   Opening: apple_firestorm_pmu/cycles/
> > >>>>   ------------------------------------------------------------
> > >>>>   perf_event_attr:
> > >>>>     type                             0 (PERF_TYPE_HARDWARE)
> > >>>>     size                             136
> > >>>>     config                           0 (PERF_COUNT_HW_CPU_CYCLES)
> > >>>>     sample_type                      IDENTIFIER
> > >>>>     read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
> > >>>>     disabled                         1
> > >>>>     inherit                          1
> > >>>>     enable_on_exec                   1
> > >>>>     exclude_guest                    1
> > >>>>   ------------------------------------------------------------
> > >>>>   sys_perf_event_open: pid 1045843  cpu -1  group_fd -1  flags 0x8 = 4
> > >>>>
> > >>>> ... which should not be PERF_TYPE_HARDWARE && PERF_COUNT_HW_CPU_CYCLES.
> > >>>>
> > >>>> Marc said that he bisected the issue down to commit:
> > >>>>
> > >>>>   5ea8f2ccffb23983 ("perf parse-events: Support hardware events as terms")
> > >>>>
> > >>>> ... so it looks like something is going wrong when the events are being parsed,
> > >>>> e.g. losing the HW PMU information?
> > >>>
> > >>> Ok, I think I'm getting confused by other things. This looks like the issue.
> > >>>
> > >>> I think it may be working as intended, but not how you intended :-) If
> > >>> a core PMU is listed and then a legacy event, the legacy event should
> 
> The point is that "cycles" when prefixed with "pmu/" shouldn't be
> considered "cycles" as HW/0, in that setting it is "cycles" for that
> PMU.

Exactly.

> (but we only have "cpu_cycles" for at least the a53 and a72 PMUs I
> have access in a Libre Computer rockchip 3399-pc hybrid board, if we use
> it, then we get what we want/had before, see below):

Both Cortex-A53 and Cortex-A72 have the common PMUv3 events, so they have
"cpu_cycles" and "bus_cycles".

The Apple PMUs that Hector and Marc anre using don't follow the PMUv3
architecture, and just have a "cycles" event.

[...]

> So what we need here seems to be to translate the generic term "cycles"
> to "cpu_cycles" when a PMU is explicitely passed in the event name and
> it doesn't have "cycles" and then just retry.

I'm not sure we need to map that.

My thinking is:

* If the user asks for "cycles" without a PMU name, that should use the
  PERF_TYPE_HARDWARE cycles event. The ARM PMUs handle that correctly when the
  event is directed to them.

* If the user asks for "${pmu}/cycles/", that should only use the "cycles"
  event in that PMU's namespace, not PERF_TYPE_HARDWARE.

* If we need a way so say "use the PERF_TYPE_HARDWARE cycles event on ${pmu}",
  then we should have a new syntax for that (e.g. as we have for raw events),
  e.g. it would be possible to have "pmu/hw:cycles/" or something like that.

That way there's no ambiguity.

Mark.