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

[Ian Rogers]

On Wed, Nov 22, 2023 at 7:49 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> 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@xxxxxxx> 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@xxxxxxx> 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@xxxxxxx> 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@xxxxxxxxxx> wrote:
> > > >>>>>>>>
> > > >>>>>>>> [Adding key people on Cc]
> > > >>>>>>>>
> > > >>>>>>>> On Tue, 21 Nov 2023 12:08:48 +0000,
> > > >>>>>>>> Hector Martin <marcan@xxxxxxxxx> 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.

This would break cpu_core/LLC-load-misses/ on Intel hybrid as the
LLC-load-misses event is legacy and not advertised in either sysfs or
in json.

Thanks,
Ian

> Mark.