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

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Date:   Wed, 22 Nov 2023 13:19:34 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
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

Em Wed, Nov 22, 2023 at 03:49:18PM +0000, Mark Rutland escreveu:
> On Wed, Nov 22, 2023 at 10:06:23AM -0300, Arnaldo Carvalho de Melo wrote:
> > 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".

root@...-rk3399-pc:~# ls -la /sys/devices/*/events/*cycles
-r--r--r-- 1 root root 4096 Nov 22 12:35 /sys/devices/armv8_cortex_a53/events/bus_cycles
-r--r--r-- 1 root root 4096 Nov 22 12:35 /sys/devices/armv8_cortex_a53/events/cpu_cycles
-r--r--r-- 1 root root 4096 Nov 22 12:35 /sys/devices/armv8_cortex_a72/events/bus_cycles
-r--r--r-- 1 root root 4096 Nov 22 12:35 /sys/devices/armv8_cortex_a72/events/cpu_cycles
root@...-rk3399-pc:~#

But on x86, on a AMD machine:

⬢[acme@...lbox ~]$ ls -la /sys/devices/*/events/*cycles
-r--r--r--. 1 nobody nobody 4096 Nov 22 12:48 /sys/devices/cpu/events/cpu-cycles
⬢[acme@...lbox ~]$

And an Intel:

[acme@...co asahi]$ ls -la /sys/devices/*/events/*cycles
-r--r--r--. 1 root root 4096 Nov 22 13:11 /sys/devices/cpu/events/bus-cycles
-r--r--r--. 1 root root 4096 Nov 22 13:11 /sys/devices/cpu/events/cpu-cycles
-r--r--r--. 1 root root 4096 Nov 22 13:11 /sys/devices/cpu/events/ref-cycles
[acme@...co asahi]$

Slight difference with those - and _.
 
> The Apple PMUs that Hector and Marc anre using don't follow the PMUv3
> architecture, and just have a "cycles" event.

I see, and even being prefixed with the PMU name, as
"apple_icestorm_pmu/cycles/" it ends up trumping that and moving that to
(PERF_TYPE_HARDWARE, PERF_HW_CPU_CYCLES) instead of
(/sys/devices/apple_icestorm_pmu/events/type,
/sys/devices/apple_icestorm_pmu/events/cycles) as I noticed with:

sys_perf_event_open: pid 0  cpu -1  group_fd -1  flags 0x8perf_event_open({type=PERF_TYPE_HARDWARE, size=0 /* PERF_ATTR_SIZE_??? */, config=0x7<<32|PERF_COUNT_HW_CPU_CYCLES, sample_period=0, sample_type=0, read_format=0, disabled=1, precise_ip=0 /* arbitrary skid */, ...}, 0, -1, -1, PERF_FLAG_FD_CLOEXEC) = -1 ENOENT (No such file or directory)

I.e.:

type=PERF_TYPE_HARDWARE, config=0x7<<32|PERF_COUNT_HW_CPU_CYCLES

It should be:

type=/sys/devices/apple_icestorm_pmu/events/type, config=/sys/devices/apple_icestorm_pmu/events/cycles

That is the minimal patch to address the regression reported, even if
using some kludge to buy time for a longer term more elegant solution,
Ian?

> [...]
 
> > 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.

And thus, armv8_cortex_a53/cycles/ and armv8_cortex_a72/cycles/ should
just fail as there is no "cycles" for that PMU, no fallback.
 
> * 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.

- Arnaldo